When I was at school I was a "Saturday Boy" at Sainsbury's (a supermarket for those from outside the UK). My responsibilities involved pricing frozen chickens, collecting trolleys (which I liked) and occasionally the checkout.
I remember one day being offered a Scottish Banknote and asking the supervisor if we accepted it. The gentleman was a little upset and commented it was legal tender, it didn't matter as apparently we did.
However Scottish notes are not legal tender in England and shops in England (and I guess Wales) can refuse to accept Scottish notes and there is nothing that the person with the note can do.
Shadow Scottish Secretary David Mundell is launching a private member's bill in the House of Commons which would require all providers of goods or services in the UK which accept Bank of England notes to take Scottish bank notes on an equal basis.
In Scotland three banks retain the right to print their own money: the Royal Bank of Scotland, the Bank of Scotland and the Clydesdale Bank. However NOWHERE in the UK are these notes backed by law. Indeed even in Scotland the only notes with legal force are those printed by the Bank of England.
On and Channel Island and Isle of Man currency is only legal tender in those jurisdictions. A right pain when you come back from Guernsey for instance.