I'm researching a book about UK hippies. I've seen a few estimates of attendance at the Isle of Wight Festival in 1970, ranging between 300k and 800k. I wonder if anyone has a definitive, or near definitive answer. I'm certain that it was the largest crowd ever gathered in the UK for an entetainment event. But where would it stand in a league table of greatest number of people gathered in one place in the UK? Presumably the Iraq War demonstration had a higher attendance, but estimates vary for that too.
It's always going to be difficult to get exact attendances for very large open-air gatherings as these are rarely all-ticket and often spread over a large area. So you're always going to have guestimates.
At very large events, the crowds often move about and people come and go at different times, which makes counting more difficult. If counts are made at different points in the event, the estimates may very well differ due to the movement of people.
A further complication for very large events is deciding where one crowd stops and another starts. For example, the London VE day celebrations may well have had 8 million on the streets, but they were at street parties located all over London, so can they all be considered as attending a single event or not?
The BBC estimate for the "millenium" fireworks show (January 2000) along the Thames in London was 3 million. Since they were all watching the same display, I would say that was the largest attendance for a single event.
It may not have even been the biggest of Queen Elisabeth II's lifetime. I've seen estimates for the crowd at her Diamond Jubliee in 2012 ranging from 250,000 up to 500,000. Its at least fair to say it was in the same ballpark.
I can't find any estimate for the overall crowd size in London on VE day (just some specific areas), but it seems nearly the entire population of about 8 million took to the streets.