Lies, you either had a lot of gear, I doubt it, the girl was doing something obscene, or this never happened at all.
So you have a small camera, the girl is fully clothed and has her legs closed?, bollocks!. So any tourist that wants to take a pic is going to be told to do it in their own home?
Maybe the caretaker didn't like being told to fuck off for asking you to stop doing something you shouldn't be. You were NOT nearly arrested for taking innocent pictures, oh no you didn't even get started, of a fully clothed girl.
As for the English and Londoners being prudes I think this might show you it's not exactly the case.
http://www.flickr.com/photos/simoncrerar/sets/442816/