Sorry RAZ, I hadn't realised you were actually here yet.
I like Neals Yard ( a courtyard off Neal Street ) in Covent Garden. There are oriental vegan buffets all over the place. There is a veggie/vegan fast food place in Soho - RedVeg. If you go to Camden market, there's a stall there that does a variety of vegan burgers. Obviously a lot of ethnic eateries will have vegan options.
Holland & Barrett is the main health food chain here. There's a few GNC shops in London too.
I am not sure where you are staying and intending to visit, so can't really be more specific at the mo?