What vidarh said. Country A can use country B's currency without any consent; lots of countries do this. Some of the more volatile countries use dollars unofficially as "hard" currency for big purchases.

I'd expect Scotland to parallel Ireland's history of using the pound and pound-equivalent: https://en.wikipedia.org/wiki/Irish_pound

See also https://qz.com/260980/meet-the-countries-that-dont-use-their...