There are 250+ vegetarian restaurants and stores in London:

http://www.happycow.net/disp_results_address.php?distance=15...

EDIT: London also has a vibrant vegetarian culture with many veggie groups. For instance, there's a very nice meetup group with almost 2000 people that convene regularly: http://www.meetup.com/vegetarian-419/

And a vegan meetup group: http://www.meetup.com/londonvegan/