My wife and I just returned from two weeks in the UK. I happened to bring along a Chip/PIN credit card issued by my credit union, and I'm really glad I did. Many, many establishments either didn't accept swipe-only cards, or claimed that they couldn't. This even though the credit card readers clearly had a space for swiping. Several times I literally had to show them how to swipe one of my non-Chip/PIN cards. Mind you, I never had to actually use my PIN, so it was more of a Chip & Signature transaction each time. What surprised me most is that London was where we had the most difficulty. So I'd just like to pass along that, at least as far as our trip is concerned, Chip/PIN cards are becoming obligatory. I realize that RS along with others claims that you can simply insist that the clerk swipe the card instead, but let's be honest: if the employee doesn't know how to process the transaction, or if you don't want to appear the rude American who demands things be done his way, it's better just to "do as the locals do" and get a Chip/PIN card before traveling.
Thanks for sharing your report. It's true that the staff person you're working with has as much effect as the real rules. Many places (Pret a Manger, etc) also just expect customers to swipe the card themselves, and the system may look different than what you're used to.
I just returned from Wales (three weeks) where I had only one restaurant insist they could not use my traditional card. In two places I taught the cashier how to use a swiped card and they seemed grateful. No problems in London in numerous shops.