It is more US than anything and the outlets deal in Dollars. IIRC one fast food outlet would accept £ but gave the change in $ & c.
I could never understand why the RAF didnt adopt the same sensible policy. A trip down the route to the far east could necessitate changing money into Cyprus£, Bahrein dinars, Gan cowrie shells and finally Singapore$, with exchange charges at each stage. Dont know what the arrangements in Afghanistan are, but once that has gone and the RAF is reduced to the size of a flying club I dont suppose it will matter any more.
