Anyone have recommendations for shopping loose leaf tea in London?

Flight home (U.S.) canceled and I have an unexpected day off in London. I would be grateful for any suggestions. A few years ago this sub helped me out when I was in New York.

I may travel for work, but forcing myself to go out and see a city can sometimes be a challenge for me. Goals help, plus I have a camera to keep me motivated not to stay in my room. :)