"Phil" wrote

Only supermarkets/convinience stores offer cashback, as it reduces the
amount of cash they need to, pay to, bank. This is only on debit cards,
probably all sorts of consumer credit rules about giving cash on a
credit card.

For debit cards they are charged per use, so it doesn't cost tem any more if
you use the card to pay for your shopping or if they give you cashback as
well. And, as you say, it saves them the cost of banking the cash. For
credit cards they are charged a percentage, so it would cost them to give
you cashback (and you'd be charged interest from day 1, whereas .uk credit
cards generally give you some sort of intrest-free period on purchases. I
don't think I've ever had to pay credit card interest.

Peter