I will answer my own question, because I just completed a booking with them. They will add 1.4% for VISA payments on the site, plus even though the pricing is in USD, the billing will come from the UK so unless you use a foreign currency surcharge-free card like I did just to be safe, I believe you will also incur your bank conversion charges. Still worked out to be a great deal for me as their rates were already lower than anywehere else, so thanks to OP.