paying for something in usd from uk account

Anthorn

I've done this regularly when I had a Santander current account mainly to pay for coins from United States Mint on subscription - I collect U.S. coins. It's always treated as a foreign transaction as if I was using my card in the country. Your bank's tariff will be able to provide info on foreign transaction charges.

If you're doing those transactions regularly you could save money by buying a prepaid dollar card and loading it when the exchange rate is favourable.