James VI also used the term Great Britain but it was his personal use and didn't relate to any reality other than the normal geographic use to describe the island. The two kingdoms of Scotland and England remained seperate kingdoms who just happened to share a monarch. I don't know about his use of 'united kingdoms' but I would guess that he was simply talking about the various kingdoms under his control. Neither GB or the UK were any official titles for any state. In 1707 the united kingdom of Great Britain is brought into being but again the official name was GB with the UK part being descriptive. In 1801 on union with Ireland the official title of the state then had United Kingdom incorporated into it. I imagine 'united kingdoms' would be used originally because that was what it was 'seperate kingdoms united under a shared monarch' whereas later on the individual kingdoms of Scotland and England (and later Ireland) were replaced by a single kingdom.

I don't quite understand what you are meaning in regard to devolution as the various pre-union kingdoms have not been revived and we remain for the moment one single kingdom. The SNP position is that if they succesfully persuaded the Scottish people to opt for independence from the UK then the monarch would remain head of state and from that date of course Kingdom of Scotland would come back into existence.

What would be interesting is how would the rest of the UK react (ie would Northern Ireland and Wales remain firmly in the union) and what the name would be? Would it be the United Kingdom of England, Wales and Northern Ireland - or do you think they'd opt to retain Britain in the name?