my sister lives in Oxford, England. She says that flying is the way to go. the Chunnel (so cleverly named ) is pretty expensive, unless bought WAY in advance. But, if you do go to London be sure to get a ticket for the London Underground that lasts all day, and hit up as much as you can. These were where i visited on my one day trip to London (I stayed in Oxford):
1. British Museum. The UK "stole" a bunch of stuff (when they colonized basically the world) and brought it back to England. This is where most of that stuff is. ie) Egyptian stuff, Turkish stuff, Rosetta Stone, you name it.
2. British Library. On display is: 2 of the 3 oldest complete Bibles in the world, Gutenberg Bible, Leonardo Da Vinci sketchbook, Magna Carta, Hand's "Messiah," diaries of VERY famous people, etc. This was my favorite.
3. Imperial War Museum. By far the most visually stunning. They have actual fighters hanging from the ceiling, and MANY actual tanks surrounding the entrance. Awesome account of every war to date, well almost every war. They conveniently left out the American Revolution... go figure... effin Brits...
best part: They are all free!