Tower Bridge. I have lived in London for years now and yet it never fails to take my breath away. I still remember the first time I ran across it and every now and then I divert from my normal routes to pop over it.
Running over Tower Bridge is often a crowded story, with Spanish tourists everywhere but if you manage to time it right you can have a beautiful run over the bridge, along the Southbank and onwards towards Battersea Power Station. On the way west you'll also pass many of London's sights - tourist running at its best!