The Romano-British were a blend of the Romanised urban dwellers and large land owners with the less Romanised rural tribal groupings. The urban societies still functioned under Roan-style magistrates and bishoprics, and the rural under kings and warlords.
The Anglo-Saxons had their own kings and chiefs who claimed descent from a variety of Biblical or other religious beings (depending on their degree and immediacy of Christianisation!) or historical characters. Significantly different, IMHO, and easily simulated using KAP.
However, as I have said from the beginning, Your Logres Will Vary. That is a good thing, not bad, and I wish you well in your campaign.