From October 15, Mastercard will charge 1.5 per cent of the transaction value for every online credit card payment from the UK to the EU, up from 0.3 per cent at the moment. For debit card payments, the fee will jump from 0.2 per cent to 1.15 per cent. The increase will benefit British banks and other card issuers, rather than Mastercard itself.
That’s from FT.
So not only EU websites will not ship to UK they might not take UK MasterCard or even no MasterCard at all.
Or when abroad they will hear / guess British and say card machine is broken. Cash only.