I've wondered the same thing. Currently I'm leaning toward not seeking a formal autism diagnosis. 

I am actually glad that I did not get diagnosed while I was still a minor, despite potentially gaining access to some accommodations. A friend who was diagnosed young but is overall fairly "high-functioning" for lack of a better term, found that his (otherwise pretty reasonable and decent) parents were much less willing to respect his choices, give him privacy, and so on than they were for his neurotypical siblings, even into adulthood. The accommodations available for someone with autism when I was a minor would likely not have been that useful to me, though it might have meant that I got diagnosed with ADHD earlier, which would have been helpful. Overall, I'm glad I avoided something like my friend's experience.

The very low-probability outcome of my being formally diagnosed with autism now which worries me is my parents deciding to use that diagnosis as reason to do something like gain guardianship over me and control over things like my medical care, despite my being an adult. It's highly unlikely that a court would agree to it on the off chance my parents would be sufficiently controlling assholes to seek it, given their likely reasoning, but I wouldn't call it impossible.

One reason my social and medical transition has been mostly smooth is that I am financially independent from my parents and live in another province, so they cannot interfere much and interfere less than they might otherwise attempt to because they see me as a competent adult who is making choices they don't like and think are wrong, someone to be argued with, criticized, not accommodated if possible but ultimately not actively hindered, not someone whose decision-making is inherently compromised by a developmental disorder and may have been manipulated into transitioning against their own best interests. If they learn of my diagnosis and believe it accurate, that could well change.

I may reevaluate if I'm struggling at work after finishing this bit of college and think I might be able to get useful accommodations, but for the moment the benefits aren't very substantial and the process of getting a diagnosis would be irritating, plus there's a real tail risk in my case.

I was diagnosed with autism when I was about four years old. About a year later the diagnosis was changed to Asperger’s Syndrome.

Pros of having a formal diagnosis:

N/A

Cons of having a formal diagnosis:

If you want to have children one day then you will face scrutiny from social services. Do not under any circumstances pursue a formal diagnosis if you are female, intend to have children and live in the UK.

If you are still in school then you might get SEN support. This means that you will receive BS therapy once in a while. It also means that your school might get extra funding if they exaggerate your deficits enough and keep you out of any gifted programs that might make you look too high functioning. If you decide not to take notes because your lessons entirely consist of the teacher reading out the contents of the textbook and you have a copy of the textbook then you may get a taxpayer-funded scribe to take notes for you. There will also be extra legal complications if your parents want to homeschool you.

You are legally protected from discrimination at work due to your autism. This is a con rather than a pro because in the Nash equilibrium nobody wants to hire you. If you end up unemployed then when you apply for universal credit you will be told that you have to apply for disability benefits instead because you have autism. Of course you are not disabled enough to qualify for disability benefits so you end up with nothing.

Once you have a formal diagnosis your qualifications have less signalling value because potential employers assume that you had extra time for your exams.

Your parents can obtain legal guardianship over you. So for example if you donate money to a charity and your parents believe that the charity is run by scammers and that all the money goes to corrupt African dictators then your parents can declare you to be incapable of managing your own finances and gain control over your bank accounts.

If you are officially labeled as an autist (or anything else), some people or institutions may use this information to discriminate against you.

Perhaps the risk is extremely low given the current British law and culture; I don't know. But how much you are willing to bet that the law and culture will remain the same 20 years later? Once you get the official diagnosis, you might be later legally required to disclose it, so this step is irreversible.

I have checked the situation in Slovakia (which of course has a very different law and culture), and there are institutions that quite openly discriminate against autists. Like, medical schools that require you to declare that you do not have any mental condition mentioned in a list (autism included), or internal rules of police departments not to hire autists. I wasn't aware of that previously. I am not really interested in police work, and I am already too old for school, but this made me update strongly against seeking formal diagnosis for my children, should any autistic symptoms manifest at them. I want to keep their options open; and I also worry about what else may happen 20 years later.

In 2019 there was a scandal in Czechia, where a high-status psychiatrist Jaroslav Matýs said in an interview for a newspaper (article paywalled) that autists are "unable to distinguish between good and evil". He then proceeded to talk about psychopaths in politics, which kinda implied, although he didn't say that explicitly, that the conditions are kinda related. (The topic of the interview was Greta Thunberg, and as we all know, politics is the mindkiller.) There was a public outrage, and the guy resigned from his position as a chairman of a child section of Czech psychiatrist association. He still insists that the only mistake he made was "talking publicly in a manner accessible to lay audience" (source). Okay, the guy was punished for his idiotic opinions, but the fact that he was previously able to hold such position while having such opinions, is disturbing. (Imagine a parallel universe, where instead of giving a public interview, he just kept writing expert reports on people like you or me.)

To address the other things you mentioned:

A formal diagnosis means you can ask for special consideration and usually get it, even if you don't strictly need it.

Do you have detailed information about what specific accomodations would actually be available for your diagnosis? (Maybe you are too optimistic.)

Many people seem to find a formal diagnosis helpful for understanding themselves (are there advantages over a self-diagnosis?) and for explaining themselves to others.

I think it is relatively easy for an intelligent and sane person to correctly diagnose themselves as having "autism or something similar". The problem is, you probably have no idea what exactly "something similar" involves. An expert could potentially tell you that you have "X, which is similar to autism, but not the same thing", or perhaps that you have "autism and X". I hope you agree that such diagnosis would be even better for understanding yourself. (An example of X could be ADHD.)

I could just explain to people that eg I don't like last-minute changes of plans. Is there an advantage to explaining to other people that it's because I'm autistic, rather than just saying it's part of my personality? 

The diagnosis implies a difference between "mere preference" or "just something you never tried to overcome", and "something that is really difficult (maybe impossible) for you to overcome".

Because, in general, it is sometimes good to tell people "you know, if you overcome your dislike for X, it will allow you to get all these things that you may like a lot". (I don't like exercising. But I do it anyway, because of health concerns.) But if I know that X is simply not an option for you, I won't waste your time talking about how useful it could be. Though, different people will react differently. Some believe that any diagnosis can be overcome by sheer willpower.

Is it socially responsible to seek diagnosis for borderline cases?

Yes. Suppose that autism is on a spectrum from 0 to 10, where 0 is "obviously not an autist" and 10 is "obviously an autist". Your worry is that it is socially irresponsible for a 3 or a 5 to seek diagnosis, because they are obviously not a 10. But if this becomes a norm, then tomorrow a 6 will also feel guilty for seeking diagnosis. And after tomorrow, so will a 7 and maybe an 8; which is probably not the outcome that you want.

Also, consider the fact that you are (I assume) highly intelligent, which means you are better able to cope with your symptoms. You can partially overcome your missing social instincts by reading literature that explains human behavior explicitly, and then applying the lessons to specific situations; sometimes you get things right. Imagine a person with the same degree of autism, but IQ 100. Their situation would be much worse, do you agree? So maybe instead of "not too autistic" you are "intelligent enough to overcome or alleviate some symptoms of autism". You may think of yourself as 3 on the autistic scale, but you might actually be a smart 6 who is able to function at the 3's level.

You don't have to be a "perfect autist" in order to be an autist; not all autists have all symptoms. Leave the diagnosis to an expert.

Conclusion: I think the best way would be to get diagnosed (also for adjacent disorders) by an actual expert, so that you can benefit from the self-knowledge... but somehow without leaving a written record of the diagnosis, so that you can deny it later. Not sure how difficult it would be to arrange that.

A diagnosed mental health disorder can be a reason for green card or USA entry denial, if it has led to harmful behaviour. This site says:

According to the CDC guidance, the mental disorders that are most frequently associated with harmful behavior are:

• major depression
• bipolar disorder
• schizophrenia, and
• mental retardation.

This doesn't mean that other disorders won't be used as grounds of inadmissibility, however. Even anxiety disorders, which many people suffer from while leading fully functioning lives, can lead to inadmissibility, if the disorder has led to harmful behavior to the applicant or others.

It also increases your risk of denial out of public charge concerns:

The public charge bar can have significant ramifications for immigrants with mental disabilities or disorders. Immigration officials are concerned that these immigrants might find themselves stranded in the U.S., unable to find work because of their disability and unable to receive or afford appropriate care and treatment.

There are also some job restrictions, e.g. people diagnosed with autism cannot join the Navy in the UK. 

I'd thouroughly research these and other possible restrictions before seeking a formal diagnosis. And keep in mind that rules might change in the future. Given the risks, I personally would need a better reason than getting more time on exams to take such a step. Maybe if I needed medication and this was the only way to get it.

I have received a formal diagnosis twice in my life, once when I was 10 and once when I was 23.

Negative impacts: none. I can disclose or not disclose as I wish. I like to tell people I'm autistic after interacting with them for a while because their surprise gives me a confidence boost. Apparently I'm smart enough to hide it.

Positive impacts: the diagnostic track was quite rigorous, including all kinds of tests (IQ, social intelligence, some qualitative stuff), and the resulting data has been useful for me to understand myself better (for example the subscales of my IQ are highly irregular).

So I'd say: do it. Mildly beneficial.

Many people seem to find a formal diagnosis helpful for understanding themselves

Anecdote: I was in this camp, for sufficiently low values of "formal." I went to a psychologist to get checked out for autism (among other things) five or ten years ago. After testing, he said that he wouldn't personally diagnose me, but that I was close enough to the line that if I shopped around I could probably find someone who would. I said that was fine -- it told me what I wanted to know.

(hilariously, I also scored rather high on schizophrenia. His reaction went something like "Okay, obviously you're not schizophrenic. Analytical personalities of the sort that take ideas and moral systems seriously just read that way on the tests. You're not schizo, you're just weird.")

I'm in the UK as well, and spoke with my GP a couple of months ago about whether I might have autism and/or ADHD (my self-diagnoses were yes and no respectively, with enough uncertainty on ADHD that it seemed worth checking). I didn't particularly think about whether it was a good idea, I'd just noticed that at some point I went from "I know I think I meet the criteria for autism, but everyone on the internet self-diagnoses as that, so" to "I know everyone on the internet self-diagnoses as autistic, but I think I meet the criteria, so". I felt a bit awkward describing myself as autistic based on that, so I went for a diagnosis.

I might be misremembering details. But he said there's not really any test for either, especially not if I'm just curious (which is why I'd said I was looking to be tested). But that for what it's worth, he thought I probably had autism and not ADHD. (I can read blog posts! (Can I finish them? «Well, I get to the bottom, at any rate.») Apparently people with ADHD can't do that. Who knew.)

My understanding is that he's wrong, and that there's a thing with a multi-year waiting list where you can get an actual diagnosis somehow? At any rate a friend told me he was on such a waiting list a while back. But I didn't feel like arguing.

So it might be super easy to get an informal diagnosis, and a massive PITA to get a formal one.

My older brother was diagnosed as a kid. AFAIK it hasn't particularly affected his life since school (I dunno if he even got accomodations in university). But he's not old enough/in bad enough health yet for your specific worries to come into play, and also his life path hasn't involved much formal employment.

Asking for accomodations can be quite nebolous. And even if they are nebolous they can be insufficient or non-responsive to your actual needs. On the collective level asking fo rsuch things makes it more routine and improves the general availability. It can also make sense to have the diagnosis so that if you choose to ask you have more bases but not ask it for everythign and gauge it more case-by-case basis whether being a ill fit for a general mold is a bigger pain than negotiating a totally ad hoc thing.

The framing of "diagnosis" is very healthcare centric. Instead of "self-diagnosis" we could also speak of "identification". When other people view have a different opinion then having an "objective measure" can make it a societal thign rather than just viewpoints of individuals. Especially those of aspergers type thing or "high-functioning" can face the problem that people that are not familiar will outright disbelieve with things like "you speak perfectly fine you can't be autistic" or "you have too good grades to be autistic". Even the low and high functioning labels are problematic because it isn't really a core question most of the time but plays pretty squarely to common negative attitudes. An alternative viewing would have how it might be important for countries to get recognised. Its not about whether it is an effective or ineffective society but rather acknowledging its existence.

A question that might be intertwined is also whether you should take your autism seriously. A person that feels the autistic traits speak to them but never really goes forward with them is likely to have it as a very personal drama and contemplation. Diagnosis is how it gets taken seriously in healthcare. But separately from diagnosis being openly autistic or connecting to autistic culture might still be worthwhile to do even if a diagnosis is forgone. Compare a person that discovers their grandfather was Irish and then starts to consider themself irish or one that has always known it but has chosen to forgot or de-emphasise it.

Autistic persons get all of the other stuff and the expression of the other stuff can be modified. This can help unconfuse diagnosticians and you are less likely to be considerer hypocondriaric. For example pain expression can be quite muted compared to experience levels so tools like "between 1-10 how is it feeling?" and loudness of screams. Focus patterns that produce special interests will also modify what depressive "lack of enthusiasm to normal activity" will mean etc. But yes a non-neurological healtcare provider (and even one whose field is neurology) might use a cached thought based on such marks. I am estimating that things are getting more understood and communicated that fear about treatment in old age dementia are not really well-founded. For those that can appear or give an understanding of being neurotypical, remaining masking if the receivement would be bad could stay in the cards ie making it easy for people not to discriminaate by imposing a literal veil of ignorance. For those that will illadvertly give off a vibe most other explanations would be even worse.

If you just unexplainedly say that "I am unconfortable with sudden changes" people might think you are an asshole or on purpose diffcult or something like that. If you say that it is because of autism then the "its nothing personal" is an easier sell. For example I have a problem with questionaries because I will answer them very literally. This means I will treat a double negation as a positive while some other people will "typo" correct it to treat like a negation. Dealing with that can be very baffling on both sides when most of the interaction is not. Asking for a correctly pharsed question sheet or filling the sheet with unintuitive data can seem as sabotaging the questionary. I would imagine that dealing with a person that have a lot of unconnected quirks could be quite confusing so providing the "arching narrative" can help there. And it can hedge against wrong narratives such as "I am just autistic, not stupid", "I am not unimaginative, just literal"

I do have a conondrum as saw an opinion of a boss that said that if a computer programmer has trouble with eyecontact they are less qualified in the jobs core activity as discussing code with other programmers or customers is part of the package. I would argue that effective communication is part of it but it doesn't need to happen via providing social comfort via eyecontact to the discussion partner. Being able to navigate and negotaite the neurotypical expectations would seem superchallening without being able to justify the unorthodox behaaviour as legimate (and effectively call out and bust the ablism in that kind of stance)

Seeking diagnosis does not pathogolise the trait I think. Compare to the worry of deepening an image of sexual deviants for getting recognised for being gay. I do think that the resources for treatment and diagnosis can be quite thin and my worry would be that those that its more optional getting it before or hindering those for whom it is quite crucial would be an issue. UK takes bride in the NHS (but also likes to complain about it). I think in US the individual more directly pays for their care so this could be a lesser issue (by seeking the thing out you also brign comparable resources).

Speaking of which getting a condition recognised only to be told that there is no cure or mitigation of it can feel like a pointless endeavour. If there are some life difficulties their connection to the neurotype can be relieving that one doens't need to worry about what all it could be. Its also a spectrum in the sense that its not a hard yes or no but the implications change for different shades of it. This might be considered improvement as trying to torture the autism away with ABA or exile from society as being a reject just basic acceptance goes a long way.