My favourites:
1. Chariots Vauxhall - Clean, expensive, a bit sterile, and like a running track that you do laps of over n over again... BUT... On a Saturday evening through to Sunday night definitely the best option if you like your chems n bareback sex
2. Sweatbox - Dirty but more maze-like, a more interesting place to wander around. Best on a Monday if you're into twinks. Younger but higher attitude crowd.
3. Pleasuredrome - relatively clean, mixed low attitude crowd, tiny dark room but always seem to get a few anon loads in there. By far the best jacuzzi out of all the ones I've tried in London.
4. Vault 139 - Not a sauna, more a small sex/cruising bar. Crowd is late 20s up, usually get one or two fit younger guys (like me lol). Great for anon bareback sex, awesome gloryholes. Cheap n cheerful. I go for the naked nights on either a Monday or Thursday.
Horny for a few loads now I've written all that out!