I made that last mistake at the large Boots in Picadilly Circue one morning when there was no cashier on duty downstairs and I picked up a couple of diet coke cherry 500 ml bottles for £2 and used the USAA "improved" (ha ha) mc with chip which we now know is chip and signature preference #1. So...I inserted the car and was told wait for attendance. Wait for attendant. Wait for attendant. No way to cancel or whatever. Wait for attendance (for a bloody £2 charge). Finally 10 minutes later, an attendant shows up, enters her code and finishes the transaction and of course made sure she compared the signature (always done in the UK no matter how small the charge). A royal pain in the rear end for no reason at all.