I could try and get a train down to Croydon, though it's a bit far for me. I'm from the Sidcup area and tend to play in London Central/South East London. I love teaching
40k players and I be happy to help... though when 6th edition comes out, you've got to learn it all over again.
I sometimes go to Oxford Street Games Workshop, and although I'm not a great fan of Games Workshop, we can meet up there. But you're after a group, and I know three that can help you:
The first is South London Warlords in Dulwich Village, the closest and perhaps the biggest group around:
http://www.salute.co.uk/
The secondary is South Eastern Warlords which I tend to go these days, located in Catford, which isn't too far from you either.
http://www.selwg.org/
The last is the Cross Gaming Club located near London Bridge. It doesn't play much
40k (warmachine and malifaux etc) and is a bit noisy, bit it's a club.
I hope this helps, and hopefully we can play one day. I would prefer South Eastern Warlords since it's a place in between both of our positions, but it depends what works for you.
Oh, one last thing. Each club has a playing fee, usually £5. But most clubs tend to require payments.