Over the last year I visited Italy, Switzerland, Germany and the Czech Republic. And I found that I could use my VISA credit card virtually everywhere. There were a few places in Germany that required cash, but they were the exception. I would use my credit card for everything and would only carry maybe 100 Euros for when cash was needed. But I found I really wasn't using cash--and I still have most of the cash I was carrying in my wallet.
But I have never visited the U.K.--this is my first time. (Not counting layovers in Heathrow!)
How is it in London? Can I expect to use credit cards everywhere, or should I expect to need a lot of cash?