Its nowhere near as bad as it was back in the '70s or so when some places devalued the other countries' Sterling by 10% before accepting it or flatly refused it. I've spend several eg Bank of Clydsdale £20 notes in pubs in southern England. I've also spent a fair bit of English Sterling up (proper) north with no problems.

It doesn't help that there are five or so different banks in Scotland issuing notes with different designs, so to spot a dud you need to really know your stuff.

Top tip - if you find yourself with a Scottish note in England, use it in a self-checkout machine.

I had a lot of trouble spending leftover Jersey pounds when i reached London. People didn’t even recognize the bills! I think a McDonalds ended up accepting them.