You see many folks misinterpreting our scant sources for the period trying to push towards a unifying high-kingship across the scattered kingdoms, citing Vortigern, and for those that accept the historicity of Arthur, Arthur himself, but this is far from the reality of the period, and is usually a flight of fancy for those trying to make Geoffrey of Monmouth fit who they think the historical Arthur was, without accounting for the fact that kingship during the period looked nothing like the later Medieval kingship Geoffrey portrays.
I think there were regionalized high-kingships, mostly arrangements among Nobility loosely held together by shared kinship, giving some limited 'State' power to combat Germanic encroachment, but it was far from an organized resistance in most periods and instances. Not to mention that prior to the mid 6th century it is extremely likely that the Germanic tribes were likely still being used as Foederati, evidenced by mixed culture graveyards in the North.
While the internecine warfare between British groups no doubt contributed to the eventual Germanic ascendance, the cataclysm of 536 leading to a volcanic winter seems to have had a massive impact on the British compared to the Angles and Saxons, with plague of Justinian ravaging the Britons further in the next decade. Why these events impacted the Britons more is limited to speculation IMO, but it does seem to have been the major impetus for even the more militarized north to begin collapsing.
When Rome withdrew, the Britons didn’t unify as “Britons” against the Saxons, Picts and Gaels
Why not?
They did manage to stop the flood of Saxons into western Britain for centuries
How did they manage this without top down unification?