Many people on my flight today (mostly foreign tourists) were bemused and disappointed that only card was accepted as payment, not cash. The cabin crew had to make several announcements to this effect as they were fed up of people ordering food and then only offering cash.
BA lost out on 5 meal purchases from the family sitting behind me because of this. It seems like they had leftover Sterling and wanted to use it up on the flight home, not wanting to purchase anything via card due to the foreign currency charges from their bank.