There’s a continuing controversy in Scotland as to whether an independent Scotland could or should be part of a ‘currency union’ with England. The assumptions are invalid; England cannot ‘stop’ Scotland from using the pound. The EU’s position is more complex – negotiations on entry to the Euro might be part of a negotiation about EU membership – but this seems unlikely to be a sticking point.
There’s another option, however, and it’s an option for the UK as well as Scotland. It’s not necessary to sign up to anything, or to be part of a currency union, to use a currency. For example, by convention oil production in Scotland, and I assume in the rest of the the UK, is traded in dollars. What’s important is to have the facility to hold, spend or trade the currency, and that’s currently not possible for most of us – the UK banks don’t let customers hold Euro accounts on the same terms as sterling. Why ever not? A rule that provided for banking in Euros would open the possibility for people to use either currency, or both, at their convenience.