Well, what can they do if they become a separate country?
1. Use the UK pound without being in on the central bank. Their money supply is not under their control.
2. Join the EU and use the Euro without being in on the central bank. Their money supply is not under their control, and they have to follow a lot of stupid regulations.
3. Have their own little currency that everyone else in the world will laugh at. The Scottish what?
They could mint a nice one unit coin and call it the “Nessie”.