I cannot see anything controversial about some Britons becoming renegades and throwing in their lot with the Saxons, particularly during the confusion of the late fifth century . However, from what I have read from papers by Richard Coates and O. J. Padel, it seems unlikely there was much acculturation if any.
Your point about Coates and Padel is interesting. I have noticed a tendency of place name scholars to be far more aligned with your view of Anglo-British relations (two bitterly opposed factions divided along ethnic lines) whereas many modern archeologists are more aligned with my view (significant movement between two groups with ethnicity being of less importance). Synthetic histories which take in bits from the various disciplines are helpful, but the downside is that synthesists run the risk of being a jack of all trades but master of none. This is hardly surprising - how could any one person have the toponymic knowledge to rival Richard Coates, the linguistic knowledge to rival Patrick Sims-Williams, the knowledge of poetry to rival Marged Haycock, the archaeological knowledge to rival Helen Hamerow, the biological knowledge to rival Stephen Schiffels, the landscape knowledge to rival Susan Oosthuizen and the historical knowledge to rival David Dumville? This is especially the case when the synthesis attempt to make broad points about a wide geographical area. It is increasingly recognised that there was significant regional variation and this brings about another layer of specialisms. The more I study, the less I feel I know.