Go to Chinatown for some great Chinese food. Stop at an India restaurant as there are tons in central London, and they are all rather good if you enjoy Indian food.

Stop at a pub for a shepherds pie if you like that type of food. Stop at pretty much any place in town for a traditional English breakfast. It is not great, but it is entirely English and worth the experience.

Best resource in London is timeout.com. You can find some great things to see and do and places to eat.
http://www.timeout.com/london/food-drink