I'd have thought that the Scot Nationalist posititon would be that, regardless of the political position and the link with England, Scotland was already a nation, as it always had been, and that the correct way of expressing this would be by referring to "the United Kingdoms".

As for what to call the rest of the present UK if Scotland pulls out, maybe they could call it "the United Kingdom of Southern Britain and Northern Ireland". Though since neither Wales nor Northern Ireland have ever been kingdoms, I'm not too sure that "United Kingdom" would be appropriate. But perhaps it'll be a republic by that time anyway.