Have you ever eaten duck? If you answered no then I suggest you march out to the closest store, buy it, cook it, and then eat it. It it the best thing in the entirety of the universe.
Last weekend I had the opportunity to walk around Borough Market. It is a outdoor food market and it is amazing. They have a whole range of food to eat immediately. There are traditional British meat pies and mash potatoes, duck sandwiches, jamaican cuisine, fish and chips and more desserts than you can imagine.
If you find yourself in London, and you have a kitchen or somewhere to store food, you will be able to explore Borough Market even further than I did. There are fruit and vegetable stands, as well as butchers and homemade bread and pasta.