Hey there, I'd check out the following thread, it is regularly updated;
http://terminal-boredom.com/forums/index.php?topic=17956.0Also, these guys put on shows in London -
www.upsettherhythm.com - they've got Strange Boys, Nodzzz, Crocodiles, Wavves etc. coming up to give you an idea of some of the things they do.
Also, it is a 50 minute train ride to Brighton from London and we put on shows there -
www.myspace.com/sexisdisgustingrecords or
www.sexisdisgusting.co.u k. They usually rule.
There are so many other promoters doing awesome stuff in and around London, just keep checking that thread/add UK bands and labels on Myspace/look for flyers/posters in Rough Trade East.
I know your question pertained to venues rather than promoters but in all honesty, there are no venues in London that I can think of where you are guarenteed to get consistantly good shows. Also, D.I.Y spaces/house shows certainly exist but they are the rarity - you are mainly gonna be seeing bands in bars and clubs here but perhaps the occasional show will pop up in someone's house. Warehouse shows are pretty big here but a lot of them have security/a bar so really, it is just a bar in a big ole' warehouse. I think my band are playing a house show with Black Time and Black Mamba Beat maybe, we all spoke about it when we were drunk so perhaps that'll come together in September/after you arrive. peace.