On the subject of Gibraltar (having visited there in April this year), I discovered that although British Pounds and Gibraltan Pounds may be completely interchangeable at an exchange rate of 1:1 in Gibraltar, they are not interchangeable back in Britain! That is, Gibraltan Pounds are not accepted as currency in Britain (as Scottish Pounds are), and it's bloody difficult to exchange them.
To date, I've tried changing my small stash of Gibraltan pounds at Citibank, HSBC, Royal Bank of Scotland and the Post Office, to no avail. They look at me as I've asked to exchange Mickey Mouse Money. So it's off to a Bureau de Change (where I'll no doubt be hit with a commission) - or back to Gibraltar (my preferred option).
So...my tip for the uninitiated - change any Gibraltan Pound notes back to British Pound notes before leaving Gibraltar. It makes no difference to them and you won't get stuck with a bunch of notes that, as one colleague remarked, make the Queen look like she's in drag.