It’s annoying actually, many airlines let you choose the currency regardless of where you are before buying the ticket; naturally you need to go to xe.com as well to verify that the rate they give you is good!
swiss I think can be like 3% different on the fx, but from experience Turkish airlines is quite accurate so I always pay in GBP to avoid the Amex rip off.
seriously Amex, in the us the card is fx free but over here in backwards uk it’s an annoying 2.99%!