Problem is, the forces that tore the Roman Empire down are also strengthened. In the OTL 4th century, the Britanniae were a breeding ground for Usurpers, more
than any other area, until at last Honorius had enough and kicked Britain out of the Empire in 410. I don't see this changing. The Pictish incursions of
the 430s will be worse without the semi-civilised tribes of the North, as those tribes will have become just ordinary Romano-British. Britain was rich, but it
simply couldn't defend itself without outsiders. The cities had been ruined by the late Roman taxation system, there was little tradition of military
service among the senatorial/landowner class, which wasn't numerous enough anyway, and those landowners would never have armed their colones. With the
Romano-British being their usual fractuous and backstabbing selves, sub-Roman Britain would have fallen apart quicker without the strengthening by resettlement
of Northern and Irish tribesmen.
One possibility, though, would be that the British wouldn't dare to rip off their Saxon mercenaries due to the lack of alternatives. But still, once the Saxons felt they were strong enough there would be grief for the British. And as OTL plenty of British leaders would side with them. (In Wessex laws until the 8th cent. Welsh noblemen are mentioned that later merged into the Saxon nobility. Not to mention the first 3 kings there having British or part-British names.)
One possibility, though, would be that the British wouldn't dare to rip off their Saxon mercenaries due to the lack of alternatives. But still, once the Saxons felt they were strong enough there would be grief for the British. And as OTL plenty of British leaders would side with them. (In Wessex laws until the 8th cent. Welsh noblemen are mentioned that later merged into the Saxon nobility. Not to mention the first 3 kings there having British or part-British names.)
