Iv just recently began airsofting as a hobby, and have seen skirmishes with 100+ people. where are these skirmishes taking place? the closer to london the better
If it was on YouTube in a Scoutthedoggie video, then you have to realise that he announces where he's going to go, which draws a lot of extra people to those sites because everyone wants to get on YouTube in his videos.
Look for well established woodland sites if you want a lot of players, urban and indoor sites tend to have quite a low player capacity. The most people I've ever played alongside was 150 on a woodland site's "free member's day" but I'm all the way up in Yorkshire.
Your best bet would be the national airsoft event in Poole, some time in August.