Barry, according to the constitution, England, Scotland and Ireland are three kingdoms united in one, and Wales is a principality.
The United Kingdom saw Ireland separate except northern Ireland. Ireland later became a republic.
If Scotland separated, there would be no United Kingdom, as Wales is, as said, considered a principality, and was not considered a kingdom under the constitution of the United Kingdom.
As Northern Ireland is not considered a kingdom, as the rest of Ireland left the Union and became a republic, without Scotland there would only be England as an original kingdom, hence the United Kingdom would cease to exist.
Now you cannot dispute this, as it is part of the constitution!
Why do you think the heir to the English throne, in the Middle Ages, became known as the Prince of Wales, and on ascending the throne became king of England?
Because Wales, on uniting with England, was not a kingdom.
And so it remained through the ages...
I wonder if at the palace they can hear the rumbling of Scottish drums across Hadrian's Wall ....
B