There's Bijan Art on Tower Bridge Road - I've never had any work done by them but I've browsed in there a few times and thay have a nice selection of frames and always seem to be working on something or other.
I can also recommend Bijan art. I have had a number of water colours reframed over the years and I always go with an open mind as to what kind of frame I want. the guy there are very good at selecting a frame and mount that suits the picture. 9/10 times i go with his first choice but he is also very willing and patient if as on a couple of occasions I have been indecisive about which to go for. He has always done a good job. No connection etc