Check out Willows Farm in St Albans (about half an hour outside London). Not just a pumpkin patch -- it's really an Under 10s attraction with animals, tractor rides, bouncy castles, etc. and the steep admission price reflects all that, so if you don't have kids, it's probably not worth it. But if you do, it would be perfect -- they're having a pumpkin festival right now and among the attractions is a HUGE pick your own field. We were there with friends yesterday and it was great... felt just like the pumpkin picking patches of my American youth. In fact, as all the kiddos hunted for just the right pumpkin, an English friend who was with me asked "is it meant to be like this?" She'd never seen anything like it!