If you are at an ATM or somewhere where the bill is presented in GBP rather than the local currency (ie. euros or dollars), always opt to pay in the local currency. Otherwise, you’ll end up paying for multiple conversions, using the ATM or vendor’s rate, which will always be worse than Caxton’s.
Being billed in Sterling rather than the local currency is called Dynamic Currency Conversion (DCC). We have written an article explaining this further.