When we write about 5-7th century kings we have this misplaced need to assign them a kingdom. We forget that when their contemporaries in neighboring kingdoms referred to them, they were usually referred to as King of Britons, King of Picts, King of Saxons. Within their people, they do refer to kings of tribes, or kingdoms. Bede refers to English kings by their kingdom, but their neighbors are kings of the Irish/Scots, Picts or Britons. He never gives those kings a region or kingdom. The Irish refer to a dizzying array of kings and their realms, sometimes by kindred, sometimes by place. Kindreds only really seem to apply to the Irish because only they seem to have settlements based on kinship with enough longevity to generate a big enough population to be politically significant. All of the other peoples of Britain likely had large royal kindreds, but they were still a small proportion of the total population they rule over. Among the Saxons, Britons and Picts, it is more likely that kings would be identified by their core region or primary fortress. I think the kings of Alt Clut and potentially Eten (Edinburgh) are a primary examples of this.
Fraser points out that we actually don’t know that Eugein (Owain) ap Beli, King of Britons, who defeated and slew Domnall Brecc of Dalriada (Cenel nGabran) in the battle of Strathcarron in 642 was actually king of Alt Clut. (Notably Owain is the next and only king of Britons recorded after Rhydderch of Alt Clut before the late 7th century.) This is especially true because his reputed brother is Bridei ap Beli, King of Picts who killed his cousin Ecgfrith of Northumbria in 685. Fraser wonders if this family didn’t originate as kings of the Miathi. Could the Miathi have produced kings that could be either British or Pictish? Possibly, there was no people more on the frontier than them. The Maithi were considered a major Pictish tribe by the Romans but had their lands incorporated within the British civitas of the Damnonii, the same civitas as Alt Clut. After the time of Oswiu of Bernicia, the Britons were reduced to a small enough area around Alt Clut that one king may indeed have formed a kingdom later called Strathclyde. Prior to the consolidation of Northumbria and its extended hegemony, fixed kingdoms may simply not have existed. There were many kings who ruled form a primary fortress and the power of these local dynasties bobbed up and down with the success of individual kings. Only when one dynasty retained control of a large region with significant hegemony over its neighbors did something like a kingdom begin to form. Now, Eugein/Owain’s dynasty did manage to retain control of the remaining northern Britons, based from Alt Clut at least part of the time, from the mid-7th century until it finally fell to a combined Scottish and Norse siege. Such a dynasty is what was needed for Strathclyde to form into an actual kingdom. The only king who can’t be linked into Owain’s progeny is Guret who died in 658, but as the Welsh triads call him base born and this is the height of Oswiu of Northumbria’s power, it is possible that he was imposed upon them by Oswiu. It is noticable that Owain’s two sons who reputedly succeed him were named Elphin (Ælfwine) and Domnagual (Domnall) suggesting that Owain’s family had intermarried extensively with both the Scots and English. Recall that Owain’s brother Bridei ap Beli is said to be the cousin of Ecgfrith of Northumbria in the Historia Brittonum.
As for kings of this kindred moving into Alt Clut, I think this is perfectly natural for any major British dynasty that grew up in this area to take Alt Clut (Dumbarton). First of all, its possibly the best fortress in the area and further away from the English. I think the most important reason for Alt Clut being so important though is its position on the Irish Sea. Sitting at the mouth of the Firth of the Clyde, it is ideally placed for Irish Sea trade. Even Stirling, if there was a fortress there in the 7th century, was largely inaccessible to trade because the English and the Niuduari Picts controlled the entrance to the Firth of Forth.
Kingship in southern Britain among both the Britons and English may have been a little different in that it may have been based on Roman administrative districts at least initially. The Romano-Britons would have been conditioned to Roman-style administration. Although post-Roman political systems could not recreate the Roman system, it is likely that they retained some Roman territorial boundaries. Many of the kingdoms of southern Britain seem to be at least initially loosely based on Roman civitas.