Non-fiat currencies are in common use throughout parts of the UK. There are the new-fangled local pounds (but with a marked 1:1 peg to Bank of England £) in Bristol, Stroud, Lewes, Brixton, etc. but the Bank of Scotland, Clydesdale Bank and Ulster Bank have been issuing similar notes in Scotland and Northern Ireland forever.
If the free market ever decided that the 1:1 peg between the non-fiat and fiat £s is no longer justified, the fun could really start...