I just received two credit card statements, one from an old Natwest VISA card and one from the Abbey Zero card. The Zero card is one with zero loading on foreign purchases. Normal cards like the Natwest one say they add around a 2.75% surcharge.
I see a transaction on each card for purchases in Euros from France that are reported on the same date. The Zero card gives an exchange rate of 1.2609 Euros to the pound, Natwest 1.204
This is a bigger difference than I expected - about 4.6%. I spent £336 on the Zero card so using it in preference to the Natwest card saved me about £16.