I've always found that the age old rule of being nice, polite, and do what they say, you'll be fine - even in london! No-one should be scared about being 'done', they tend to just move you on like anywhere else in the country. It's when you get riders being mouthy to them, and not following simple instructions that you get the problems. Unless you're at more risky spots, such as St. Paul's, The Barbican etc (only been there the once, for about 2 minutes). I'd recommend you stay well away from those places, they're getting a lot hotter on it recently and more eager to issue fines if you hand around for too long, especially when they're trying to move you off that spot. Just make sure you stay vigilant, and everyone watch each others backs. If you see police, stop what you're doing immediately if they might be coming over to you. Just use some common sense and look out for each other, you'll all be ok.