@Addemup
First rule of AH : if you want to do a timeline that you know you'll enjoy making a sharing, implausibility or ASB isn't an issue. There some people that may think ASB means "bad quality", "poorly done", or too convenient, but frankly that's a stress on a conception of allohistorical plausibility which is dubious at best. The distinction between a very implausible timeline and an ASB timeline is sometimes foggy enough to be nothing else than assumed implausibility or being in denial about it, and I enjoy more a well-written timeline that accepts its historical implausibilities (up to a point,altough supernatural TL can be fun) than something pretending to be totally plausible because reasons and it's not ASB how dare you say, etc.
Really, go for it.
Now, if we're talking about allohistorical plausibility...We have to look at late imperial and post-imperial Dumnonia which covered both Cornwall (which was probablynot just part of it, but an autonomous constituent) and Devon. Clearly Cornwall was the backwater part of the kingdom/principalty, with few Roman military or civil presence (to the point that, while provincial, the region may have been more or less autonomous as a significant chunk of western Britain. Cornwall seems to have more tied up, culturally and economically, at Brittany than to the rest of Britain, something that definitely increased with the fall of Dumnonia which not only cut Cornwall from the last insular regions with a significant post-imperial Roman remains, but launched a new wave of migration to the continent, making an already poorly inhabited Cornwall even more so.
I don't want to be negative, but that doesn't really looks well, even for a regional revival.
As for the second part of your question, if I understand you well enough, about the possibiiities for Britto-Romans to take back the lead in Britain up to the point being the cores for a new mediterranean-based empire?
I'll extensively use
Britain After Rome there : if you don't have it, I certainly advise it to you.*
There's a wall of text incoming, so if you want the short answer : while Britto-Roman dominance in post-imperial Britain is perfectly doable, a Britto-Roman empire isn't really so.
Eventually it all comes down to the nature of roman rule in Britain. We know that romanisation was, rather than a systemic acculturation, a more or less deepened creolisation of provincial society based on political integration and an extensive trade and use of material culture (what we could call a "Roman-way-of-life"). Some people were latinized in the process (such as in Gaul), some were romanized along their own organised network and cultural structures (basically all the eastern Romania) and for some it was relatively more limited.
It was the case of Roman Britain that, up to the early IVth century, was essentially a military province for what mattered Rome with a significant urban/latifundar romanisation happening mosrly in the South (
and not everywhere in the South) while most of the roman structures in the province depending from military presence for exchanges, subsides, etc. Would have the empire fallen in the Third Century crisis, post-Roman Britain would have significantly less structures inherited from the Empire, being closer to Illyricum on this regard.
Still, things changed a bit after the IVth century because Brittonic society really began to romanize itself due to the need to compensate for the end of heavily militarized provincialism : more and more local products were exchanged, cities were less monumental but practically tought and well-maintained. Basically, a crisis managment that, so far, did work out without making Roman Britain sort of Roman Gaul expy, but his own things with pre-conquest structures (which never really disappeared, especially in peripheral regions) being integrated and integrating imperial feature.
It didn't last this long : Britain was targeted by neighbouring peoples such as Scotti or Pictii. These weren't newcomers but confederations of Gaelic and Brythonic peoples that appeared one century before, both to defend themselves, and to manage long-range raids. Let's say they weren't happy with the lesser ammount of their subsides so far regularily payed by the Empire and as
Rome withdrawed troops from the island to make up for the lack of manpower on the continent, well, they raided the heck out of the province.
Note that it's possible that you already had Saxons in southern Britain, as foedi or laeti in the Litus Saxonnorum (Saxon Shore). Robin Fleming disagrees, but I'm not really convinced by the arguments : it was common enough in the IIIrd/IVth century Gaul, so I could really see a Saxon coast guards against Saxon piracy, a bit like Normands in 911. Anyway.
It thus happened that Britto-Roman society was significantly weakened, and eventually Constantine III took with him the last regular troops, and Rome had no choice but to say "Well, you're on your own now.".
This being said, you certainly still had militias in Britain, probably with some comitatenses and more-than-token cavalry. But eventually, the only direct authority was gone, and you had a mosaic of municipal authorities, generals and capitains turned warlords, peasant communities and big landowners.
This alone was pretty much destructing but Britto-Roman society still had contact with the Roman state, notably by its presence in Vth century Gaul : Riotomagus (probably more of a title than a name, I'll come back to this) had an important strategical and military role in Northern Gaul up to the Battle of Déols.(470),
But the collapse of the Roman state in the west was another taken shot : let's remember that this fall was felt with particularly destructing
effects up to Scandinavia where appeared all the signs of geopolitical anarchy and renewed warfare. Britain basically lived trough two fall of Rome.
At this point, this much is clear archeologically, the Britto-Roman society as I described is in ruins, not just trough raiding but by sheer exhaustion and relative inability of the elites to exchange with the continent. Still viable ruins, granted, but ruins nevertheless.
When Germans came "in masse" (relatively wise), they didn't as much ignored or fought Roman structures that they didn't found much of these.to being with : most first groups of migrants (coming, in all probability, from Saxony and Anglia, but including Franks, Jutii, Norses, Frisii, etc.), or at best found them while they were collapsing. While some probably came as foedi, against Picts and Gaels, possibly against Armorican Saxons as well; most were coming as familial communities as Slavs did in the VIth century eastern Europe; not caring at the latest of collapsing imperial structures.
It doesn't mean that Britto-Romans were doomed politically and culturally from the Vth century onwards, but their division and the deep societal crisis (a definite rise of rural communities, led by landowners and/or warlords, with a quick decline of old municipes) requires as much an original and regionally tought change as it happened historically in the IVth century. Meaning an imperial ensemble or structure in the Vth century is probably out of the reach of Britto-Romans.
What we could search for is to stabilize the high-kingship features that reappeared in post-imperial Britain (if they ever entierly disappeared, which is frankly a fair question) : we can name some important rulers such as Riotomagus (which means, more or less, great king), Ambrosius Aurelianus (who was maybe, probably IMO, the same as Riotomagus) in the south and Coel Hen in the North.
From these mix of military command and political leadership (probably rather regional in nature than provincial, if Vortigern's rule over Cantium is any indication), we'd have to strengthen them to last against various and conflicting interest and identities (not unlike what existed in pre-Roman Gaul). Such a maintained high-kinghip as existed in virtually all the known medieval (and ancient, altough harder to determinate) Celtic societies, could hold its ground in face of Germanic settlements that basically made their identities and political ensemble on the spot and became centers of acculturation of the weakened local population due to a better participation of their (relatively egalitarians) elites to the wider world and trade. From there, a fair process of acculturation of newcomers could follow.
It's not as far-fetched as one could think, and it might have been what presided to the appearance (or at least emergence) of the kingdom of Wessex historically : first kings of Wessex had clearly Brittonic names, their political/palatial centers quite far from known main Germanic communities at the exception, for me, of the aforementioned Saxon Shore and its foedi. It's perfectly imaginable (and imagined by several scholars) that, the House of Wessex came from Britto-Romans warlords, princes, kings or even high-kings wannabees that based their rise to regional supremacy on Saxon troops already present or incoming thanks to promise of sweet, sweet revenues; and with the known process of cultural mixity and acculturation to what eventually became a Germanic Saxon identity (which again, is a product of the changes of the VIth century, and not something that caused them).
While I'd rather see, as I said above, at best two high-kingship ensembles roughly divided by a Norwich/Bristol line (possibly more, and the line in question being very technical), probably a Gaelic presence as well and more or less integrated newly founded tribal kingdoms as for Germans.
From there, I don't see why Britto-Romans couldn't assume (if I don't think it would be completed easily and quickly) the dialectical process of
chiefdom/complex chiefdom/state formation that, among other exemples, defined what Anglo-Saxons kingdoms went trough and possibly earlier, eventually ending as one or two coherent polities. But I don't see it happening in the VIth or VIIth century myself.
So while a Britto-Roman Empire, let alone one that goes back to the Med, seems far-fetched to say the least; you can change significantly the history of post-imperial Britain in favour of Britto-Romans.
For people interested, besides the aformentioned book, I really strongly advise the wargame
Pendragon whom author really managed to simulate the issues and possibilities of sub-Roman Britain, politically-wise.