I love Planet Organic. If you live in London they will deliver to your home as well. The one on westbourne grove has a really good hot food selection with a wide range of fresh salads and well cooked vegan foods.
You can also order from the Organic Delivery Company or Abel and Cole.
I have to recommend some amazing vegan restaurants as well!
Manna, in Primrose Hill is entirely vegan: http://www.mannav.com/
Saf, has a location in Oldstreet and a smaller location upstairs in Whole Foods on High Street Kensington. Commended for it's sustainable ethos. Raw and Vegan! http://www.safrestaurant.co.uk/
Those are my two favourites!
There is also
Vita Organic, a small raw vegan cafe in Soho and a cheap eat. I love the raw vegan cheesecake: www.vitaorganic.co.uk
Mildreds. They are vegetarian, rather than vegan, but have a lot of vegan dishes: www.mildreds.co.uk/
Food for Thought: a small cafe restaurant in covent garden always has vegan options including things like vegan scones, vegan fruit crumbles and stir-fries.
There are loads of other ones as well. I definitely think Mana and Saf are worth trips regardless of where you live in London.