You seem like the type who likes to plan ahead, that's very good!
I live in London, but I have never been to any TS clubs yet, but I have been doing a little research. There are a few clubs out here for this sort of thing, and I have found a few online.
You could try checking some of these sites if you haven't already:
Lacy Ladies
Trans-MISSION
The Angels
Good luck with your search in London, just to remind you... keep yourself safe at all times (wear a condom), if you ever find yourself in doubt about something or someone just say "no" and walk away.