Hmmm.
http://en.wikipedia.org/wiki/Banknot...pound_sterling

How quaint.

I can only assume that the Bank of Scotland and other private banks have the right to print pund sterling because an Act of Parliament says so. Which means it can be revoked.

Scotland could conceivably hold on to the printing machines and print them anyway; printing what is essenitally the currency of the remnant of the UK without their permission, but that wouldn't be very nice of them. I'd guess that there's a treaty against that sort of thing but I can't be arsed to look it up.