Peppercorn's seems to have almost everything. Even if you can't find what you're looking for, just ask a member of staff, and the desired item will likely turn up in a corner or over your head. They carry all the Demeter brand tofus, and Soja Sun yogurts, which are favorites of mine. They also have tempeh in the freezer, which I can't find anywhere else in the area. Plenty of sea veggies, Asian ingredients, toiletries, fresh bread, and healthy ready-made meals as well. They also sell homemade sandwiches and treats, of which there are several vegan options, and handy for lunch if you're nearby. They're very close to the West Hampstead tube station.
Cons: You sometimes have to dig, to find what you're looking for!