The Royal Vauxhall Tavern aka RVT is probably one of the oldest and best known LGBT venues in London. There's a bar, there's a stage, there's cabaret, comedy, drag, theatre, discos and even bingo. Check the website for what is going on. There's an entrance fee for most events, however, bingo nights (non-Bank Holiday Mondays) are free entry.
Royal Vauxhall Tavern, Kennington