If we could go back a few years - well, I am talking of more than a thousand years (!) - we would find that Britain was made up of various kingdoms, and even of peoples from quite different origins.
Now - whether it was entirely through the efforts of Alfred the Great is open to conjecture - the notion and desire was that the land should be unified. To begin with it was merely the land of the Angles (England) that became unified (after some struggle) so that when William the Conqueror took over he found a country that was well-established as a single kingdom.
Gradually, other parts of what we know as the UK became annexed - firstly Wales and then (much later through royal succession) Scotland.
For some three hundred years there was more-or-less a concensus that the union worked well. And then a loud portion of the Scots decided that they'd had enough of being ruled from Westminster and thought that independence was a good idea, motivated largely by the thought of the oil …