Athelstan himself was the end of a bloodline.For an apparent history buff he knows very little about the lineage of our current Royal family - Æthelstan was a Saxon, bloodlines probably changed at least a dozen times since then
I am pretty certain there’s more Scottish blood in our late Queen than there is in Russel.
Anyway, the first King of Britain was when the King of Scotland also became King of England and combined them so Scotland took their throne.
Scotland has also never been a republic. Even nats say they would keep the Monarch as head of state which many of them like to ignore.