In fact, topically, this is what Spain is now saying, explicitly saying it would welcome an independent Scotland (although it's also a dig at the disunity of the UK) as long as it is done "legally": https://www.independent.co.uk/news/world/europe/brexit-news-spain-northern-ireland-border-scotland-nationalism-catalonia-josep-borrell-a8642476.html
DGR - according to ds, Keith Brown (SNP Deputy Leader) said at a meeting last night at Aberdeen Uni that the SNP would announce their latest Indy currency choice at their spring conference.
Personally, I had no issue with sharing the £ - at least initially. After all, people didn't say that Ireland wasn't "independent" because the punt shadowed precisely the £ for over 50(?) years.
Also there are many countries that use the US $ (some deliberately, like Panama, others less positively because their own currency has collapsed, eg Zimbabwe) that are nevertheless independent. As I say, if people had bothered to listen to Mark Carney (for whom I have a lot of respect), they'd have heard him saying that it was possible. Even now, it is arguable that the UK Government has "ceded sovereignty" to the BoE, when it made the BoE independent of government
. So it's ok for the UK Government but not for Scotland 
It is similar to the Eurozone arguments: the Eurozone countries had to cede an element of sovereignty to the Central bank when they joined the Eurozone but no one, except the diehard Eurosceptics (
), claims that they're not independent countries 