Watching John is at the very TOP of the list. In London, if you have a free night, and can do it, I suggest seeing Oliver! It's an amazing production. (Especially if you get Burn and Rowan still in it, I'm not sure how long they'll be on stage for the show, though)

In London, there is just so much to do. If you've never been, go to the Eye. British Museum is nice if you want to be inside for a while (if it's raining), um... Covent Garden. There's so much that you're going to have to pick and choose. London is a wonderful city.