London is great and all, but my preference is country and small towns.
I have to say on that trip my favorite place was the Imperial War Museum. My father traveled to England a few times on business, and had wanted to go to the Imperial War Museum, but never made it. He had asked me to go see it, and I did. Not to give out TMI, but my father is no longer with us, which made the trip especially important.
Normally though, my favorite place would be the Bluebell Railway. I was there also on my first trip over.