There is precisely one issuer of non-Mastercard/Visa credit cards in the UK - Amex - and they're not accepted nearly as widely. There are exactly five banks which issue Mastercard debit cards rather than Visa - Virgin Money, Citibank, Clydesdale Bank, Yorkshire Bank, and Metro Bank - and none of them are particularly major players. (I think it was down to three or less at one point; Virgin and Citibank only switched in 2015.) Santander and TSB have apparently announced they're planning to switch to Mastercard debit in the future but currently their customers have Visa debit cards too. Maestro is dead as a dodo here, and the old UK-specific Switch debit card system no longer exists.

So in actual fact, people in the UK who're reliant on their debit cards are more likely to be affected by this than people with credit cards too, even if the cards are from the same bank - banks that issue Mastercard credit cards generally use Visa for debit cards.

95% of debit cards in the UK are Visa according to the FT.