The first time I was in Britain I had difficulties paying with Scottish money in London (long ago).
But just two years ago when I brought back one pound of each of the Scottish banks for the fun of it and went to a German bank to get them changed they made it three different transactions, one for the Clydesdale,..... This way they could charge threee times the changing fee.
I never could get changed bank notes from the Faroes outside of Denmark, though this money is accepted in Denmark.