I am confused by the quick turn-around in the UK, here are 3 scenarios I considered and why I am not convinced by either.
a) Herd immunity reached as the last peak infected (and made immune) the remaining people. Problems: Intuitively this would have to have been very targeted infection to cause such a quick turn / high change in immunity, as we have millions, 30% population  (10% adults ), without antibodies. We had a few hundred thousand infections maybe, so I would not expect stark changes in effective R from this.
b) Last peak & immunity in clusters of unvaccinated, or otherwise more-spreading ("party"), people? So similar to the above scenario but much more inhomogeneous, and if we just make the part of the population that is "driving" the pandemic (assuming such a part ot the population exists) immune that might lead to the decline. E.g. (i) in age-groups (young), or (ii) social circles of unvaccinated people (surely there is a correlation between your friends being vaccinated and you being vaccinated), which could be small enough so that the peak indeed corresponds to running out of people to infect? This should be possible to check, by (i) age-distribution or (ii) vaccination status of infected although I have not found that data
c) Could a significant decrease in testing make the cases appear to be lower? The positivity rate  dropped from ~ 11.8% (19th) to 9.6% (24th, most recent) while daily cases dropped 41.8k (19th) to 37.1k (24th)  --- the positivity rate dropped actually more than cases but the peak of testing (19th) was after the cases peak (43.5k on 21st), which confuses me, though the ratios between the respective peaks and 24th match. Testing rates went down but the data is rather noisy .
PS: Metaculus  prediction for large 3rd wave this year (>250 deaths/day) went down to 22% from ~ 40% 10 days ago.
Note: I checked the data & wrote most of this 3 days ago, might be slightly out of date now. Decline in UK seems to flatten now.
 https://ourworldindata.org/covid-vaccinations https://coronavirus.data.gov.uk/details/vaccinations https://ourworldindata.org/grapher/uk-covid-positivity?time=2021-02-04..latest https://ourworldindata.org/grapher/uk-daily-new-covid-cases?yScale=log&time=2021-05-11..latest https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/conditionsanddiseases/bulletins/coronaviruscovid19infectionsurveypilot/previousReleases https://www.metaculus.com/questions/7537/250-deathsday-in-uk-third-wave/ https://coronavirus.data.gov.uk/details/testing