I found so many great tips here before (and during) my most recent UK trip this Fall that I felt it was only right to give back even in a small way.
Visit the Tate Modern on a Friday or Saturday, start late in the afternoon, and when you are done, proceed to the top floor and enjoy the view!
If you are nursing a cup of coffee or tea at the moment, feel free to read though an account of my experience there at http://ithinkiamgettinglost.blogspot.com/2008/10/tate-modern-and-why-i-love-getting-lost.html :)
Happy travels!
- Joel