vegan in london
The VeganLondon site is a good place to start. Take a look at their 'where to eat' section:
You will undoubtedly spend some time in Soho on your trip - maybe check out the new Vegan Routes bus on Brewer St? They say: 'Our unique menu invites you to savor flavors from all over the world: Italian farinata, African chilli with coconut, Caribbean stews, plus familiar favorites like jacket potatoes and vegan shepherd’s pies.'
If you like Japanese food, consider Itadaki Zen - fairly new and low-key, it specialises in vegan sushi, tempura and other Japanese dishes.
There are some general vegetarian restaurants listed here - check out Saf and Vanilla Black for dinner.