[threedaymonk]

As a customer, there aren't many circumstances. The only one that springs to mind is when buying airline tickets with a credit card (but not a debit card).

Where it makes more of a difference, though, is that the fee charged to the merchant is a sufficient disincentive that it's pretty common for small shops and bars to refuse card payments under £10. This is even true of the new(ish) contactless payments that only work up to £20 and which were touted as replacing cash.