Which part of LOndon are you in, there are good dive shops in most parts, so no need to travel miles.
7mm wetsuit - fine if you dont intend to dive all year round, or are really hard.
Seriously if you intend to dive al lyear then its a dry suit.
Are you a member of a club, how much diving experience do you have, have you tried UK diving at all.
The reason for asking if you have tried UK diving is that if you havent but think you might then you need different kit (eventually) than what you'd buy for warm water diving. UK regs need to be cold water rated, they arent a lot more expensive, unless of course you buy warm water rated ones first (which many of us have done of course - hind sight and all that). There are other things too, but it may be a little too soon to launch into a "what you need is" respone until we know a little more about your intended route through diving (you may not even know yourself yet).
Matt