The main issues with revolts in Late Roman Britain were that they weren't secessionists at all, but tentatives to claim the imperial dominance over Romania, or at least a very important part of it.
Just as the so-called Gallic Empire wasn't a sign of a Gallo-Roman proto-nationalism, if Carausius's Empire was more successful it would have gone for Gaul and other parts of western Romania.
At best, you'd have a de facto division of the Empire in different ensemble either as during the IIIrd century crisis, probably (eventually) as a more divided post-Tetrarchy Romania. But that wouldn't be remotely looking as a Romano-Britton state, but as a Roman polity in western Romania including Britain.
Eventually, the big problem with Late Roman Britain is its relative backwardness compared to other provinces of the Empire, its political divisions (tribal structures still largely existed, and mixed a lot with roman administratives structures, up to largely influencing them).
A true Romano-Britton polity would certainly be post-Imperial, and more looking like sub-Roman Britain ensemble.
These, in particular, may have a better chance to survive than IOTL.
After the roman withdrawals (407 was basically the last drop), you didn't have a roman army to speak of, meaning no general, critically when early Vth century WRE was far from being "beyond repair" (it really began being so by the mid and latter part of the century).
Saxons didn't pop up and conquered all the island the moment the province was abandoned
(altough they probably were present along the Saxon Shore as settled coastguards), and while the provincial structures vanished (being based on imperial authority) the Britto-Roman cities/tribes were maintained and formed entities of their own.
Having, however, an unified entity defining itself as Roman isn't much likely. You had only a more or less superficial sense of romanity in the Southern cities, and virtually absent from half of the province.
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, after that Coel Hen/Caelius Votepacus died. (Altough that might be a good PoD).
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.
Anyway, the survival of one or two Britton states (one in the North, the other in the South-West?) that would keep more of areas more touched by Roman influence could survive as post-Imperial entities.