Apologies for bumping but this was part of my musings on the Carolingian Empire (the other being a Tetrarchy like division).
I think the 700s is a little too late but I see one way to get some of the Saxonshore:
The Britons better weather the Plague of Justinian slightly so that Angle and Saxon (etc) colonisation is slower though Britons still settle Armorica.
->
OTL Devon is still part of Cornwall/Dumnonia and not Wwstern Wessex
An ATL Offa's dike is further East.
The Northumbrians do worse against the Cumbrians so Strathclyde is bigger, a rump Rheged survives on the west coast, and Chester is Caerleon under Rheged or Gwynedd.
With a weaker Wessex, Mercia and Northumbria end up the dominant Kingdoms for longer. With Mercia looking north, the Saxon Shore Kingdoms become more linked with the Franks so when Mercia and Northumbria merge they join up with the nascent Frankish Empire to avoid assimilation.