ms. cupcake is an awesome vegan cupcake shop in Brixton if you're in the area.
My favourite restaurant by far is Itadakizen near Kings cross which does amazing vegan sushi but it is a bit pricey and you need to book for the evenings.
Also at Kings cross is Vx which is a small vegan shop that also does burgers etc which are good.
I also like the Black Cat in Hackney which is a vegan cafe. There's a new place called Temple of Hackney that's opened up nearby that I have yet to try but it seems to be getting a lot of good reviews.