I don't think Romanitas in Britain would survive. On the contrary, Romanisation of Ireland and the Lowlands would only ensure that it falls quicker. Before
I elaborate this, a quick aside. The Rhine is a cultural break today. French has spread eastward due to French conquests from the XVI century onward, but there
were some Romanic pockets throughout the German-speaking areas west of the Rhine, for example along the Mosel. As for the Hadrian's wall and the
'natural' border of England, that was established comparatively late. Edinburgh, for example was in Northumbrian hands from the early 6th to the 10th
century. The Lowlands and Borders were constantly in flux between Brythons (Strathclyde), Scots, Picts and Anglosaxons.
Late and especially sub-Roman Britain depended on Mercenaries and Federates for their defense. Of the former, tribal Irish in the West, especially Dyfed and a few other, smaller Kingdoms, mutated in a few generations into a thoroughly Brythonised warrior aristocracy. In the east, the Saxon (or Angle) federates rebelled and plundered wide areas. I won't mention Hengist and Horsa in that context, they are mythological ancestor-figures and of comparable historicity to Aeneas or Brutus of Troy. But the most important source of military strength were the barely romanised, but christian tribes between Hadrians and Antonine wall. They were the muscle the Britanniae brought in to contain the Saxons, which they did. In fact, they transformed the whole Britain in the process. By Gildas' time, the 2nd half of the 6th century, educated Britons were able to write in a very good Latin, superior to writers from, say, Frankish Gaul, but their worldview was Celtic, with nary a shred of Roman mentality surviving. And they boiled out of Britain to conquer and settle in northern and western France, far beyond the borders of modern Brittany and even to northern Spain.
What a lasting Roman rule over Ireland and lowland Scotland would accomplish, is ruining those areas economically during the late Empire, as well as leaving them unable to defend themselves, far less the rest of Britain. The Britons, by lack of alternatives forced to import more Saxons than OTL, will be conquered in the 440s instead of the 590s. As for the rest, while Brythonic left little traces in the English language, in the veins of 60% of the English flows British blood.
Late and especially sub-Roman Britain depended on Mercenaries and Federates for their defense. Of the former, tribal Irish in the West, especially Dyfed and a few other, smaller Kingdoms, mutated in a few generations into a thoroughly Brythonised warrior aristocracy. In the east, the Saxon (or Angle) federates rebelled and plundered wide areas. I won't mention Hengist and Horsa in that context, they are mythological ancestor-figures and of comparable historicity to Aeneas or Brutus of Troy. But the most important source of military strength were the barely romanised, but christian tribes between Hadrians and Antonine wall. They were the muscle the Britanniae brought in to contain the Saxons, which they did. In fact, they transformed the whole Britain in the process. By Gildas' time, the 2nd half of the 6th century, educated Britons were able to write in a very good Latin, superior to writers from, say, Frankish Gaul, but their worldview was Celtic, with nary a shred of Roman mentality surviving. And they boiled out of Britain to conquer and settle in northern and western France, far beyond the borders of modern Brittany and even to northern Spain.
What a lasting Roman rule over Ireland and lowland Scotland would accomplish, is ruining those areas economically during the late Empire, as well as leaving them unable to defend themselves, far less the rest of Britain. The Britons, by lack of alternatives forced to import more Saxons than OTL, will be conquered in the 440s instead of the 590s. As for the rest, while Brythonic left little traces in the English language, in the veins of 60% of the English flows British blood.
