I would echo the suggestion of the Crystal Palace site. We went there last year and it is a great and extremely well equipped site. If I had one gripe it is not about the site but about the time it takes to get into central London on the bus, I guess the overground may have been a better option or we might tale the bikes next time.
We went just for a Saturday night the other weekend and because it was a flying visit stopped on what we believed to be a safe residential street where the parking restrictions were relaxed at the weekend, we sort of knew the area because a friend lives down that way its near Queens Park but I think that half of the beauty of a California is that you can street park, sleep downstairs and this can be done without drawing attention to yourself, then hop on the nearby Tube and Bob's your uncle, staying in London and camping for nowt.
Fin