English banknotes aren’t legal tender in Scotland. Scottish notes aren’t legal tender in England or Scotland.
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.
Both pages also go on to argue that legal tender has little practical significance.
But people do in fact attach some significance to this, with the result that Scottish banknotes are less widely accepted than they would otherwise be.
And so, why does the UK not simply declare/legislate that these notes be granted the status of legal tender? What are the costs of doing so?
A similar situation also exists with Singapore and Brunei currency (which are interchangeable), with the similar result that Brunei currency is often not accepted in Singapore. See e.g. this 2017 story.