Ive flown with FC a lot and find that it works out better for both parties to use english sterling when buying stuff.
They sometimes say that if you use foreign currency they only give you back change to the nearest euro or whatever and not small coins ie cents etc if you follow.
P.S. I noticed that they sold a phonecard for a tenner, a underwater camera for a fiver, a anti mozzie keyring thing for a fiver and Britney Spears Midnight fantasy for 18 quid.and SJParker Lovely approx 21.00 quid and a Kylie Darling for a similar price.