If you have one of these UK cards that charge 3% for foreign currency conversions, you pay the 3%. And nothing else.
The cards I have charge 0% and under 0.5%.
However, I still prefer to use my Kasikorn account and cash in Thailand for most small transactions. If booking hotels with an app and prepayment, I use the Halifax Clarity Credit card, which charges 0%.
The other thing that's prevalent in Europe is retailers offering to charge you in your home currency. Refuse that as the exchange rates are horrific. I've not yet seen this in Thailand.