I tried to buy some small thing in The Link (a UK phone shop chain owned by Dixons) and the clueless assistant said that none of my offered credit cards were acceptable becaue they were not chip and pin cards. He didn't try to run any of them, just rejected them on sight.
I explained to him that chip and pin is not used in the states and that as his shop displayed the Visa symbol they were required to accept all valid Visa cards, including foreign cards without chips, but he didn't believe me.

I checked the Visa website later, and it confirmed that he should have accepted my card. Of course by then I'd already bought the thing I wanted from the other phone shop 2 doors down with no trouble at all.

Still I found this a bit worrying. It shakes my confidence in credit cards as a source of payment in the UK, which I visit all the time.