In reply to Spike:
Personally, I love HarroWall - really big centre, lots to try, but it's quite a long way from Central London.
Yonder is very funky and has a nice comp wall.
Substation in Brixton is great, the 12m roof is a lot of fun.
VauxWalls are good but can get busy in the late afternoon/evenings
EustonWall - the one time I;ve been was very crowded with the afterwork crowd and I wouldn't go again.
I'm partial to Arch North which always seems quiet and has nice setting, again a bit of trek from central.