My late husband, who was a Scottish dance band leader, maintained that the cultural border between England and Scotland was the river Tyne, not the river Tweed.
That would put Northumberland in Scotland.
Either would suit me.
Well, both Cumbria and Northumberland have been part of Scotland in centuries past, so I suppose theres an historical precedent for it.
Getting used to bagpipes, porridge and haggis might be an issue though!
But the southern border of the old kingdom of Northumbria is actually the river Humber ...