Further to usage of 'Scotch' to describe anything vaguely northern (but mainly literature), even as late as 1969 it was being used deliberately by Hugh Trevor-Roper in his very influential book 'The Invention of Scotland'. He uses it to emphasise how Scottish culture was over the centuries invented, whole imaginary royal lineages, early pseudo-history, the appropriation of the kilt (an Englishman's invention), the bagpipe, the word Celtic etc.; still being used to stir up Scots' move to independence.

In the 17th century when it first replaced the word 'Northern' to apply to literature and song, it actually meant anything written in a pseudo style in London but being set in rural areas anywhere north of Watford, or attempting to imitate something that might have come from there. I read a lot of 18th century London literature (unfortunately 99% of it is the absolute pits) and I frequently come across the use of the word 'Scotch' in the titles being used in this way.