I know it sounds cheesey, but the open bus tours are pretty good. My girlfriend and I took those tours in several European cities (to start us off on the site-seeing), and the London tour was the best. But that's also because we were lucky to get a good tour guide. He was pretty funny, with a dry sense of humor, whereas others are probably just going through the motions.

* side note *
If anyone takes an open bus tour of Berlin, don't take the German/English one. Our tour guide spent about 80% of the time speaking in German, and always at the best times. She made the English speakers on our bus completely miss the Berlin Wall.