[Lalwendë]

I'm not sure its the Romans. Tolkien says "occupying lands that once had been part of its domain", which would suggest a culture that had retreated and might suggest Britain after the Romans had left. But he also says "living in contact with", which would not suggest The Romans. Not only had they left by that time, but the Anglo-Saxons pointedly avoided anything which remained of the Romans; they were deeply superstitious of what remained and would simply not contemplate living in the old villas (as the Britons were quite happy to do. Anglo-Saxon culture seemed to wholly deny that the Romans had even existed. So I don't think it was the Romans.

But it could have been the Romano-British. The Saxons took over land that had belonged to these people, pushing them back to the edges - 'lands that had once been its domain' and they lived 'in contact with' them by marrying their women (the Anglo-Saxons forbade British men from marrying during the early days at least).