Not to be a downer – but don’t expect too much of the tea in London…
Despite its global reputation for tea, there is not much to be found for real tea enthousiasts, unless you like Builder’s tea (Brick-red, strong, sugary tea with milk) or the posh version (very light, greyish). I myself fancy a cup of builders tea from time to time for sentimental reasons, but it is definitely not easy on the stomach!
I would advise to avoid Twinings. At least buying tea there, although an old name, it has become very commercial tea, quality is not very good. They smell nice, but flavours all dissapear once brewed. The original location is interesting though, it is the first teashop in London, and has a tiny museum attached.
Fortnum & Masons tea collection is slightly better (and pricier), but still not much to be had for real enthousiasts.
And then you have some chinese tea shops in Chinatown, which mostly sell standard teas from China. There is one large Tea/Chinese medicine shop at the Corner to Shaftsbury avenue, but they are known to rip people off, I would not advise to buy there.