Honestly, I would argue that the Britons circa AD 500 are a Roman civilization: Fluency in Latin to extraordinary degree at the height of Late Imperial style, evidence of Roman legal style education (St Gildas exhibits stylistic usages reminiscent of late Imperial rhetors), the widespread existence and adherence to the Roman Religion-Christianity, construction of large structures in late Roman architectural styles (the hall in Viroconium) granted in wood not stone, Late Imperial military organization and tactics albeit on a (much) smaller scale, the possible maintenance of the Roman legal system (some evidence exists in a few of the Llandaff charters).
What do you define as 'Roman civilization'? If you mean a purely political continuity, that already happened IOTL. There were loads of small "Roman" polities in Britain from 400 CE pretty much until the last of them were absorbed by the Normans nearly a millennium later.
If you mean a military continuity, that is pretty much impossible. Britannia was never that useful to the SPQR - its conquest was politically motivated to boost a few emperors' prestige. And the Crisis of the Third Century hit the islands hard, wiping out most of the Roman progress there. By 400 CE, the Roman colonies had collapsed, and the island is the posterchild for the Decline and Fall narrative.
Here (and only here), there was a genuine and almost total collapse of urban civilization. This is the one part of the empire where life really did become nasty, brutish and short when the dark ages hit. If there ever was a Mad-Max style apocalypse in human history, Britannia circa 350 to 550 would be a top contender.
The reason the Romans pulled out because there really wasn't a whole lot left to protect.
Yes, I am thinking of a Roman successor state, that speaks Latin, with all the tech of ancient Rome that could possibly use the resources in Wales. The coal, the iron and water. Isolated at first but survives and if ever it would expand.
The Roman forts in Penydarren or Caerleon or Nidum/Neath creates a successor state that assimilates the Welsh population with literacy(reading, writing), education, citizenship. Roman Engineering still existing and continues rather than declines. Like Roman roads, roman buildings made in local stone, with concrete mixed with limestone instead of volcanic ash from Sicily continues to be built hundreds years post 500 AD.
It would be mostly a Roman state with some Brytonic/Welsh elements.
Essentially a Roman successor state that takes over the Silures or the area of Glywysing.
So, by the time 7th to 10th century happens, Kingdom of Morgannwg is essentially a Roman state instead of a Welsh state. So by that time, with people and resources, could they used the resources of the area like Iron which was essential to the army or a Roman Legion continuing to be armed and supplied.