Them having ancestral ties to the Britons is wholly irrelevant unless you think the Celtic language, and identity is embedded in their genetic memory. Anglicized Britons didn't think of themselves as Brythonic, and wouldn't have held the customs of the Britons. Furthermore the Anglo-Saxon population in Southeast Britannia was not insignificant, and had a much larger genetic impact than any other Germanic migrators of the period. It's why their language and culture completely replaced the native one, rather than them being subsumed as the Franks and Goths were in their conquered territories. And the Suebi in Spain are not at all comparable to the Anglo-Saxons. There was something like 50,000 Suebi at absolute max in Spain, and they weren't all heavily concentrated in a single community. By contrast Anglo-Saxons makeup a huge share of the population, and have a large, continuous, cultural sphere separate from the Celts. Maybe they wouldn't be "Romano-Germanic" but some kind of "Anglo-Roman" or "Romano-Saxon" in Southeast britain is far more likely than the "Romano-British" culture of urbanized Celtic speaking Britons, especially given the historic Roman practice of using native aristocrats as their means of assimilating conquered peoples. The Romans would prop up Anglo-Saxon chiefs in Anglo-Saxon paguses, not Celtic ones, to be their clients and to promote the cooperation of their new subjects/citizens.