Gnarls - I wasn't a vegan when I lived in London, but looking back the BEST food i had in that city BY FAR was the indian food. It was simply amazing everywhere i went. I bet you could find places that use oil instead of ghee or simply ask for a substitution - and you will have the best meals.

Have a great time!

where are you going and when?

Ok so my favourites are:

Mildred's - definately top of my list.....not entirely vegan, but lots of vegan options (if you go their mushroom pie is so so so good - GET THE PIE).  It's in quite a good location too - near Picadilly Circus.  A downside is that you can't book and sometimes it feels a bit like you're being rushed.  Although last time I went they had just opened a cafe restaurant (not able to serve alcohol) just a couple of doors down which was great - three of us had the place to ourselves but didn't feel like we were putting anyone out or fell uncomfortble at all.

Also, a bit more price but yummy is saf but that's a bit more out of the way and in not a cool area.  A really nice restaurant though and all raw and so yummy.

Or actually maybe my favourite is 222.  Everything is vegan and it's well priced and a nice quaint cafe type atmosphere - I managed to take a big group of friends there (all meat eaters), when I left my last job and they loved it too.  Again this is a bit out of the central area though and in a not that nice section of London.

Eat & Two Veg is also quite good, but not really anything too special in my eyes.  A bit like a diner, but good burgers.  Not as many vegan options -especially lacking vegan desserts, which always upsets me!

Manna is nice - I think that the atmosphere is a bit more upmarket than most other veggie restaurants I've been to.  I went there for my birhtday last year and the food was great - especially the vegan chocolate fondant.

Rootmaster is quite cool & quirky and the food was really good when I went (especially the vegan chocolate cake!).  However, we had really bad service (although that may just have been bad luck) and no toilet on board - we had to use the nearby pub!  Worth a try just for the fun factor.

The Gate is supposed to be amazing (from all the reviews), but when I went I wasn't too impressed - I thought it was quite overpriced.  It also didn't help that they forgot that I ordered the vegan option of one dish and acted really put out when I refused to have the ordinary vegetarian one!

A great vegan cafe / bar if you go to Camden (rather than eating dubious stuff off the market) is Inspiral.  They have a really good selection of food.  When I went you could get a box of food for about £5 and they just kep filling it up and up for me until I said it was full enough.  Their chocolate brownies are drool-worthy too.

Carefule of the Chinese buffet places - there are a few around that are completely vegan and about £5-£6 for an all you can eat meal, but all the ones I have been to have been so so salty, so for me not that pleasant and not a nice experience.

Any way I think your question was any must go places (I got side tracked) - I would say try Mildred's if you're in central London or Inspiral if you're near Camden.

How long are you going to be in London for?  Where abouts are you staying?