We have clearly left my start date of 1066 behind – to the tune of some five hundred years – but nothing happens in a vacuum historically speaking: territories and politics evolve.
Hadrian’s Wall marked the border of the Roman Empire but by the fifth century things were looking grim and there was a proliferation of military based kingdoms. The kingdom of Rheged could be found in modern day Cumbria extending into the Eden Valley and Westmorland. It’s ruler Urien or Urbgen can be found in twelfth century Welsh poetry. One of Taliesin’s poems refers to him as the ruler of Aeron which might be Ayr – meaning that on the flimsiest of circumstantial evidence the Romano-British kingdom of Rheged could have extended from Ayrshire south of the Solway. The inhabitants of the kingdom spoke a Celtic language and its rulers were descended from Coel Hen – that’s ‘Old King Cole’ to you and me and I doubt very much whether he was a ‘merry old soul.’ Needless to say there is a lack of paper evidence and after Urien’s death, probably at the hands of one of his own extended kinship network, the kingdom disintegrated and was subsequently incorporated into the kingdoms of Strathclyde and, more definitely, Northumbria.
In the early medieval period, which was the Dark Ages when I was at school, the British kingdom of Strathclyde covered the area, at various times, between the Clyde and as far south as the River Lune in Lancashire. It’s thought that the kingdom derived from a fourth century state that was a buffer zone designed to hold off the Picts from Scotland and the Scots from Ireland (they settled in Argyle).
Now we throw the Angles into the mix. Æthelfirth was the king of Bernicia (think Bamburgh) and of Deira (think East Riding and North Yorkshire). His period in power was 592-616. He was a successful warlord who gained significant territories at this time. It’s likely that Rheged disappeared into his power and that the Lothians also came under his control. The Venerable Bede paints a picture of ravaged Britons. At the same time as Rheged disappeared Strathclyde also faded for a time.
A succeeding king – Edwin of Northumbria- even had an impact on the Isle of Man. Northumbria became the most dominant of the early medieval kingdoms during the seventh century. The territories around it shrank or were subsumed. It was at this point that the Northumbrians probably sought to establish overlordship over the kingdom of Strathclyde which had undergone some shrinkage since the second paragraph of this post. Bede also records that some Britons who lived in Strathclyde looked to the Picts and the Scots for support. Inevitably after the initial bonhomie, the Britons of Strathclyde faced danger on two fronts. In 711 and 717 the people of Strathclyde were defeated by the Scots. The area Bede was describing included Dumbarton, Renfrewshire and Lanarkshire. The Solway probably marked the edge of the kingdom of Strathclyde at that time. And needless to say there was an awful lot of slaying going on. By 750 the Northumbrians had annexed southern Aryshire.
During the 840s Kenneth Mac Alpin united the Scots and the Picts. The royal families of the region formed alliances, intermarried and carried on bumping one another off. The shape of their territories changed and developed according to who was handiest with their army.
And because I like a date to pin these things to – this all happened before 875 (or thereabouts) when Bishop Eardulf of Lindisfarne fled with the body of St Cuthbert as a result of the arrival of the next set of invaders – the Vikings (but that’s a different story and a new post.) As the saints body was kept at Whithorn in Galloway for a while it has been suggested that the area was still part of Northumbria at the time – certainly there were earlier monastic affiliations which meant that the saint was welcome.
https://sourcebooks.fordham.edu/basis/bede-book1.asp. or https://www.gutenberg.org/files/38326/38326-h/38326-h.html
Tim Clarkson, The Men of the North