"The history of Northumberland is interesting, going back to the ancient 'kingdoms' of Bernicia and Deira, the former of which went a long way into what is now Scotland."

I think you're really meaning that Northumbria rather than Northumberland stretched into what is now Scotland. Northumberland being the English county of the second millenium and Northumbria being the earlier ancient kingdom that at one time stretched from the Forth to the Humber.