you can change Sterling for Dirhams when you buy something in a shop at the airport. They always give you change in Dirhams, so don't buy something for £1 with a £20 note unless you want £19 of UAE money. What I usually do is change £10 or £20 into Dirhams at M&S or the Post Office and then spend it on refreshments on my outbound and inbound legs. TBH I've travelled so much in the last few years, I've got loads of different currency stashed at home, the chances are that I'll use it all eventually so I don't bother changing it back.
