In reply to Elena:
Hi Elena... Welcome of course, I am a member of Herts & Essex climbing club which is based at Harlow climbing wall and we climb in and around London. Regularly
The Best climbing wall to me is The Reach in Woolwich great people relaxed plenty of variety all the way from top rope slab climbing up to lead overhangs.
http://www.thereach.org.uk/
Or you have The Castle Climbing Centre which is again a great place to go its in the Finsbury park area
http://www.castle-climbing.co.uk/
Any questions please let me know
Ian