It's all Mastercard/Visa now in the UK, for both debit and credit cards - and I think Visa is probably the more common of the two these days. Debit cards are pretty much always Visa here and have been for about a decade now.
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.
Anyway I got declined with my visa debit card earlier, tried another payment terminal in the supermarket and it worked fine.