I believe this Delta collapse. It's exactly what you'd expect given our pace of vaccinations.
You're right though that the numbers somewhat suggest that we're managing to beat the Delta variant. Is it due to vaccinations? The vaccination rate has been slowing for months now, yet the UK still had that spike. The difference in people that decided to get vaccinated between early June (start of positive cases spike) and July 21st (when the decrease set in) is 9%. (from 59% to 68%). It doesn't seem to me that a few percent more vaccinations explains everything. A percentage in the upper 60s, for first vaccinations no less (not even a full vaccination), should not be enough to beat Delta from what I heard about the R0.
(Data source is the same OWID link, you can configure it to show vaccination data.)
The reason I say this is that we have a relatively small population of unvaccinated people, and so with the high R0 of Delta, you'd expect it to rip through these unvaccinated pockets quickly and burn out, which is what we're seeing.
I'd expect a few more Delta mini-waves similar to this in the coming months.
Okay that's easy to check: data should show a reduction in positive tests roughly on par with the reduction in number of tests.
Recent peak value of tests per day was 16.42 per 1000 citizens on Jul 17th. At that time there were 630.57 positive test results per million citizens.
On July 28th (latest data) tests declined to 13.46 and positive cases to 443.45. That's 18% fewer tests and 30% fewer positive cases, ballpark twice as much.
I don't think your gut is telling you the full story.
All values are 7-day rolling averages to avoid weekday spikes or anything. Source: https://ourworldindata.org/explorers/coronavirus-data-explor... (use the selector to switch between tests, positive cases, or other metrics)