This won’t happen at a point of sale, only online.
This is a huge problem, particularly in Spain. When you use a UK-issued card, including Curve and Revolut, card terminals of many Spanish merchants, ranging from independent restaurants to supermarkets like Mercadona, encourage you to transact in GBP, converted from EUR at a poor rate or with a huge markup. You always have the choice of paying in EUR, but it’s often not the default option. Paying contactlessly, e.g. Apple Pay, can prevent this malpractice, although some card terminals disingenuously allow the merchant to change the currency and amount after you have authenticated the transaction contactlessly. I haven’t yet tested whether this can happen with Apple Pay, as Curve didn’t support Apple Pay when I was last in Spain so I used my German debit cards directly.