If you're riding through the City of London (rather than the rest of London), it has its own police force who tend to be no fun. Would recommend lights if you're in the square mile, as they're known to pull you over and have a word.
If you ride on the canal paths, technically you're supposed to have a horn/bell and a license. But no-one has these so...
Oh and teat the large articulated buses and any heavy goods vehicle (dump trucks etc) with some respect. They're generally driven by idiots and on the narrow streets of London, they're reet dangerous. Quite a few folks get squashed by 'em.
Aside from that, try heading to the Duke of York on a friday night (any night is good, but friday is always a little crazy). London also has several critical masses (
http://www.criticalmasslondon.org.uk/).