My wife and I went to London last summer for our honeymoon. Some of the things I enjoyed the most were:

The Tower of London, it's just and interesting building with interesting history. Be sure to get one of the tours given by the warders. The warder we had was great.

Windsor Castle: Why? Because it's just a really cool castle.

Stonehenge: This was interesting and once there, it doesn't take too much time to do.

The town/city of Bath: Basically this is a town that was built right on top of a Roman town that noboby knew was there. You can actually go down into the baths that they've excavated, which is very interesting.

We actually took a day tour that took us to Stonehenge, Windsor Castle, and Bath. It was expensive, but worth it.

If you do go to London, one thing I would stay away from is Madame What's-her-face's Wax museum. It was 50 pounds per person and quite boring. I did the whole thing in 45 minutes.