If this happens, you should kick up a big stink. If it’s a restaurant, then don’t be quiet about it. Complain to the waiter that the restaurant is scamming you by charging your card with more than you authorised. The only way to deal with this malpractice is to be vociferous about it, and turn it into a big problem for the restaurant. Insist that they reverse the transaction and charge you correctly, and if they take their time, insist on a free drink while you wait.
And if you’re not from the UK, then also make the point that GBP is nothing to do with you and that your card is denominated in another currency. Ask them why they charged your card in a currency with which you have no connection. Make them feel embarrassed about it.