Long before my trip I purposely got a chip and PIN credit card from Andrews FCU. So when I landed at Heathrow and went to the Harrods store I excitedly pulled out my card. I stuck it in the slot on the little wireless terminal, it whirred and flashed and ... out popped a receipt to sign.
Undaunted, I then used the card in various locations in Switzerland and France. Each time, same thing - stuck the card in the chip slot and I had to sign the receipt. So apparently the card is set to default to signature. The one time I might have needed the PIN was at a ticket machine in the Paris metro, but the machine was acting up and I thought it wiser to just push in my euro coins and be on my way.
Every other time we used any of our credit cards there was no problem. Your mileage may vary of course.