Large supermarket chains like Sainsbury's have uniform training across the UK which mostly works (barring local pockets of endemic idiocy like some I've met with in London this week). Some of them have large bundles of standard procedures summarized on separate cards that the point of sale staff can consult, and you can bet "how to swipe a foreign VISA card" and "how to tell an Amex holder to bugger off" will be in there.