Check and see if Aardman studios is having an exhibit anywhere in town. I don't know if they still do those after the fire a couple years ago, but they had some pretty amazing stuff to show.
Check out the British Library to see if they have any exhibits. They had Tolkien's works on display when I was there (but not anymore I think), plus a lot of cool historical documents like music from famous composers and the Magna Carta. Bonus- it's really close to King's Cross station where they have a sign for Platform 9 3/4 (and shopping cart coming out of the wall). I also really enjoyed the British Museum.