Wanted to touch on this real quick: assuming the Romano-Britons DID have that unity (or at least, more than OTL for any appreciable amount of time), where would you place the Anglo-Saxon area of settlement at? I'd think either East Anglia or Kent, but would they really have both (or neither, being somewhere else)? My own gut feeling is that the former would be the case.
Being no expert on the matter, but having just read by
Britain After Rome, I get the impression that the Romano-Britons having that unity is actually a pretty big challenge. Fleming gives the impression that Roman Britain utterly collapsed, almost on its own, in the aftermath of Roman withdraw, with the already economically debased form of life that existed in 410 more or less gone a generation later (a thought I find fascinating: She makes it clear that Britain in 340 was still pretty much classical, with relatively large cities engaged in production for long distance trade, with those urban centers as homes of an elite with 'Romanized' attitudes, but that this situation changed fast and, by the turn of the 5th century, it was replaced by a still economically sophisticated but much more localized and 'small town' oriented market economy. Elites moved into suburbs that were becoming more and more like rural manor houses, with the leftover cities becoming more and more agricultural in orientation. Even this was gone after the Romans left, however, with old urban centers pretty much abandoned and elites concentrating in new, fortified strong points and focusing entirely on local society. I can just about imagine someone born in the 340's living to a ripe old age in the 430's and having an experience something like someone in a post-apocolyptic Hollywood movie). The Anglo-Saxon invasions (and this is part of where the author's thesis gets weird: Her take on the 'invasions' is that they were more like 'new neighbors' than marauding barbarians....yeah) only came maybe another generation after that.
The collapse of Roman Britain was more comprehensive than just the removal of the Roman Army, in other words, it was a totalized social collapse, central authority didn't just wane, it completely disappeared, and the kind of social structures that you might build a replacement on also ceased to exist. Might be why sub-Roman Britain experienced an actual, literal Dark Age where we have practically zero extant writings and literacy may have dried up almost entirely, versus the situation on the Continent, which never got that dire.