The most expensive thing was lodging. I discovered the i booth in the train stations. In almost every train station there's a booth with a little "i" logo, where you can tell the person how much you want to spend and what kind of lodging you want, and they'll find it for you. (King's Cross in London didn't have this, but they had a LastMinute.com booth, which was adequate.) In York, we told them we wanted to spend the price that would have ordinarily gotten us a B&B farther from the center of town, but they found a 3 star hotel for that price right in the center of things.

Food can also be pricy, especially with the high exchange rate. What I did the last time I was in London (in August, so it was warm), was buy food in a grocery store and eat dinner in the park. Many Londoners also do this, so it wasn't strange.