They have a great list of restaurants in Marylebone and are constantly adding new options. Just type in your post code so see who delivers. They are growing in lots of other areas in London - in fact I am a bit jealous of some of the options that people closer to Soho and Notting Hill get!
Having used the service, I can say that the food arrived hot and fresh and just as delicious as when I eat in the restaurant, and you can track your order in real time. No more rubbish take-aways for us!
(I am writing this totally unprompted by Deliveroo - we used it as we had a flyer through the door and I was just really impressed by the service and idea)