I'm a bit further out than that (Bromley/Sidcup way) - so if you have a car...

Near me is Ruxley Manor (Sidcup way), which has a good range of plants and products. Also - if it's still there - just the other side of the Rotherhithe Tunnel (i.e. North side) has a great little garden centre, with lots of relatively cheap pots.

Sorry I can't help any more than that.

Zoe