The Heaven nightclub used to be where people went before it was taken over by G.A.Y. and became "trendy" (i.e. Americanised). It is next to Charing Cross station, under the arch. People like Kylie Minogue perform there and it is dirt cheap, about 3 quid with a flyer.
There are too many clubs to mention. Most are dance, but there are leather clubs, biker clubs, fetish clubs and various other types that you can find via the web.
Soho has become the main area, it is London's answer to Greenwich Village. People openly mince down the street and nobody raises an eyebrow.
It's really best to do a search on the web, pick up some of the papers, like Pink and Boyz, or find a gay haunt and ask where people go to do whatever it is you like doing.