First stop...London!
What could I say about London? Maybe that even if I never thought of it as a must see, I would gladly go back any day. There are many attractions in London, it all depends on what you're interested in. What you shouldn't miss is the Changing of the Guard at Buckingham Palace, Tower of London, the parks, Trafalgar Square, The Monument.... and if I don't stop soon, I'll probably list almost all the attractions.