So Indo-European people have lived in Britain for at least 10,000 years. For at least 8000 of those, there was no concept of England, Wales or Scotland beyond regions of a map. They were 'Britons', tribes of Celts (although 'Celts' were not a people per se, just a culture, like 'Western' today doesn't mean just one ethnicity).
People migrated around constantly, the tribes all knew each other, and there's evidence every tribe in Britain met up at Stonehenge, walking from as far away as North Scotland every year to see their tribal cousins.
Then between 55BC and 1066AD, the island was invaded by Romans, Scandinavians, Saxons and Normans (the last 3 are all Germanic). Various regions of the island warred against each other and the population mixed all throughout this time until the Anglo-Saxon was the everyman in most areas, remote areas were mostly still Briton and the upper class was mostly Norman. The British Empire became the most powerful empire in history and spread its language to every corner of the Earth.
So why do Scottish people think they're different to the English and vice versa? Why do we see ourselves as different nations? I don't get it. They're literally just regions with the same blood but slightly different histories, different accents and that's it. Just like places like Yorkshire, Cornwall and the South-East have different accents, histories and fought against each other in history. Why is that a basis for being a different nation when we all share the same blood, overall history and language? One faction was bound to conquer the whole island at some point, and obviously it was going to be the most populous region that did so.
Is it football that has spread this D&C? Apparently St George's flag was almost forgotten until it started being used for English football. In the days of Empire "England" was used to describe the entire Island. Scottish or Welsh nationalists hating England now is like me hating people from the old kingdom of Anglia because I was born in what used to be Wessex, and now the government happens to be in Anglia.
100% of British people have the same history for at least the last 500 years (being beaten and ruled by Westminster), the same overarching history for the last 10,000, the same blood, the same broad culture, and the same first language. Why do we see ourselves as different nations?
My family name is a Viking one, which comes from Yorkshire but I've also found it cropping up in Edinburgh records. I look 100% Norse and so do half the people I see wherever I go in the country, and I've travelled around quite a bit.
I live in Scotland and I'm sick of Scots treating me like some oppressive foreigner when we literally are the same people with a different accent.