The best place to start is with the NME small ads - they have a section devoted to rehearsal rooms, and you may be lucky there. If that doesn't work, call a couple of venues near to where you want your room, and ask for contact numbers for local bands. A call to them, and you'll probably find they rehearse in the South London area, because mosty bands rehearse and play in their local area. That route should hook you up with someone with a room and some time, so you can make a booking.