I'm in Glasgow but would heartily recommend dropping by TeaSmith, the owner John is always happy to meet a fellow tea addict. I've not been to visit Postcard Teas yet but I've had some sublime teas from Tim there and from emails back and forth with him, the man is passionate about his teas and their sources.
Some London based tea ramblings from Chadao may be of interest:
I am from France but I try to go at least twice a year in London. I love this city. When I am there, first thing I do is go and pay a visit to Tim. If you are looking for enthusiast tea people, you'll sure like him. As mentioned, Teasmith is a hot spot too where to have good tea discussions.