I went to London last year and have a few suggestions for some great restaurants:
Purezza in Camden: Great pizza, pasta and desert like Tiramisu
Unity Diner in Whitechapel: Amazing vegan meat and cheese (burgers, fish and even steak)
Oowee in Brixton: Not really a restaurant but a fast-food joint. Imagine a completely vegan McDonalds.
If you like fast food, you can also go to an actual McDonalds, they have a great vegan burger there that's only available in the UK (McPlant). Enjoy your stay!