As the subject, really.
Alex the Salmon says that, post-independence, Scotland may retain Sterling. Westminster, in return, says "No, you won't. We won't let you."
But - can they? Surely any small country can declare their official currency to be anything damn well want, without having to have "permission" from that currency's usual home.
The Euro's used in several non-EU countries, including Montenegro - I can't believe that the ECB has said to the Montenegran gov't "Yep, you feel free." So there must be some parallel mechanism going on, with absolutely no way of controlling it.
At the very outside, there could be a Scottish Pound officially set up, with an exchange rate pegged £1 = S£1, I s'pose.