As a former inhabitant of London, I can offer you another couple of tips for eating in London...
Wagamama (4 Streatham Street off Bloomsbury Street, London WC1A 1JB) is a Japanese noodle bar which also serves some veggie things. The atmosphere there is great (everyone sits together on huge tables) and best of all, it's cheap! Get there early or you will queue forever.
I can second that Crank's are a nice spot for eating (there is one in Covent Garden, one near Trafalgar Square, one near Goodge St). Also, Food for Thought (31 Neal St. in Covent Garden area). With all this talk about sushi lately, a fun place to go is Moshi Moshi Sushi (upstairs in Liverpool St Station) sushi glides by on a conveyer belt and you can ask the chef to whip up some veggie sushi for you (very crowded at lunch time)
Kinda fun is to go to www.fodors.com and search their restaurant guide (you can specify vegetarian cuisine)