A fortnight ago as I attempted to pay for groceries in England with a Royal Bank of Scotland £20 note the cashier kept putting it in a machine and then told me that she didn’t know if she could accept it.  It was the first time I had ever had trouble trying to pay with a Scottish note.  I read today that a pharmacy group have decided to stop accepting Scottish banknotes because they are some of the most counterfeited in the country.   They acknowledged that although they are Legal Currency, they are not Legal Tender, leaving it to the discretion of the retailer as to whether they accept them or not.
The Committee of Scottish Bankers confirm
Scottish Banknotes are legal currency – i.e. they are approved by the UK Parliament. However, Scottish Bank notes are not Legal Tender, not even in Scotland. In fact, no banknote whatsoever (including Bank of England notes!) qualifies for the term 'legal tender' north of the border and the Scottish economy seems to manage without that legal protection."
£5 plastic banknotes are now being introduced in Britain offering more enhanced security features, durability and cleanliness than the paper £5 note.  A new security feature has been incorporated in the form of a transparent window which changes colour as the note is moved and tilted. The Clydesdale Bank was the first to introduce them with limited circulation in March last year and then more general circulation at the end of September with The Bank of Scotland and Royal Bank of Scotland introducing them in October.  The Bank of England also introduced a new £5 polymer note in September and I have had a few of them but not yet had a new Bank of Scotland note.  It will be interesting to see what happens if I offer a new Scottish note when I visit England next week.