This is a restaurant with a view. This rooftop restaurant will give you views of Nelson's Column, The Big Ben, The Picture Gallery and St. Martin-in-the-fields church. It's a bit expensive but if you choose well you can get away with a 2 course menu for £18. Also good atmosphere and friendly service.
-- Second pic. from despitz (flikr) under CC --
phone number: 020 7306 0055