An unified state in post-Roman Britain would have be near impossible, IMO.
Cross-posting from
another thread.
By "Romans" I assume you mean Britto-Romans, rather than either only military (that was anyway quite mixed itself, up to local and germanic recruitment). See, you didn't have real provincial identity in Roman Empire, and even the few you may have was even more weakened with the more personal and charismatic imperialship of the Late Empire.
With the decline and fall of this one in WRE, the basic identitarian structure was the pagi, the colonies and critically the tribes (either classified as cities or pagi themselves) whom presence never really disappeared.
This isn't just a British thing, it was the case for almost all WRE provinces : Gaul, Spain, Africa (constant overlapping of tribal and urban/peri-urban identities)...
The big exception being Italy, mostly because it passed directly from Imperial to Romano-Barbarian dominance.
In all these cases, the Barbarians that inherited the imperium and political legitimacy eventually were the unifiers of the dioceses. It's not about Britto-Roman or Roman population being too dumb to unify and to live, but about political and cultural trends of Late Roman era.
You'd probably need a PoD in late Republican Rome to deal with the relativly empty shell that were Provinces when it came to identity. At latest, you'd need something comparable to the Sanctuary of Three Gauls, with its technically unifying yearly assembly. For the influence it had eventually on Gallo-Roman society in Late Antiquity, I'm not too sure it would be ennough to balance the aformentioned effects.
Eventually, if you want to bypass the provincial issue, you have to come back to the usual problem of Late Roman Brittania : isolated, not that deep romanisation (lack of many urban infrastructures, safe in an handful of cities; lack of rural infrastructures, with a low ratio of villae/latifundiae), demographically weakened (maybe 1 million after the epidemics).
It doesn't mean that you couldn't have ended with Saxons being repelled as unified political entities, but you'd need a twist.
We know that they were unifying commands (I'd tend to argue they were more regional and circonstantial than pan-Briton : as Vortigern for the Cantium), at least military-wise : Riothamus/Ambrosius Aurelianus (possibly the same person) is an exemple. So the problem isn't having unifying features, but to make them last against the various and conflicting identities.
It doesn't seem, for instance, that the Old North kingdoms had a much develloped sense of commonity, for all we know.
Now, I think it's possible to have a maintained high-kingship (pretty much as in Ireland, Wales or Scotland) in some regions. Giving the not that much unified Anglo-Saxon kingdoms, it does have a chance to lives on and leading to a wanked *Wales. I don't know enough to definitely name a candidate or a most likely place (while I think that you can forget about North Sea regions); but the bonus point is that you don't even need a Britton or being totally hostile to Germans to have such.
Cerdic of Wessex may be the most obvious exemple of a mix of Britto-Romans and Barbarian elements in the Vth century (you have other ones). It wouldn't surprise me if you could have a Britto-Roman high-king, supported by the Saxons of the Litus managing to lead a more or less unified (in a first time : again, high-kingship didn't looked much as a really united structure) Britto-Roman kingdom.