Difficult to know what kind of crowd you'll get without the right promotion, the same venue can sometimes be packed and sometimes be empty. In London you could look at the Half Moon, Camden Underworld, smaller venues like Fighting Cocks in Kingston, here in Brighton there's The Hydrant (I'd try and play a free show downstairs, upstairs is boomy), Hobgoblin which is small and pleasingly dingy, Prince Albert which has amazing sound but is difficult to get, latest musicbar is a good place though you may have to play slightly quieter than usual. Hope that helps a little, let me know if you need more help!
Also add Adam Fillary on facebook, he puts on a lot of metal gigs round the South of England and might be able to help you, tell him Kit said he might be able to help out.
Also, my band plays hard rock too, check it out, let me know if you want to share a bill. We are not a big band at all, though!