According to Robin Fleming in her Britain After Rome the "de-Romanized" status of Britain is more accurately described as de-urbanized. Roman identity in Britain did not vanish overnight with the advent of a new Germanic culture across the whole of Britain, and the Romano-British people, or Welsh, or Britons, are evidenced throughout the Anglo-Saxon migration to England. Famously the far west of Great Britain, modern day Wales, resisted "Anglo-Saxon" encroachment, and Latin and Christian enscriptions are attested from western Brtian after the collapse of formal Roman authority.

What caused this however? It was not a simple matter of the Anglo-Saxons sailing to Britain, slaughtering all the native inhabitants, and setting up their own communities. Britain in the fifth century was going through a dramatic economic collapse that utterly devastated the urban centers of Britain and resulted in the abandonment of these cities. This economic collapse also resulted in the removal of Britain from the Roman trade system, and from the fifth century on traditionally present trade goods, such as Africa pottery, start to disappear from Britain.

Despite this, and the influx of Germanic (and Irish, and Pictish) migrants into Britain, Roman identity did not vanish in Britain, and various small scale polities that sprang up across Britain in the immmediate post-Roman period did maintain claims to Roman legitimacy or what is termed "Romanitas", through both the use of Latin and Christianity. In lowland parts of the island though these communities were eventually displaced, or absorbed, into a hybrid culture that we call Anglo-Saxon.

However, aspects of Roman life in Britain, the villas that dotted southern Britain, the infrastructure of the British Church, trade goods from all over the empire, urban centers, and so on, DID vanish over the course of the fifth century. Different scholars point to different causes though, as I mentioned, Robin Fleming points to economic collapse and a subsequent migration movement that resulted in a new system of life that drew from both antecedents, but also went through a more or less anarchic period before polities started to solidify a century or so later. Peter Heather posits that the migration/invasion of Britain by the Germanic people was on a much smaller scale than on the continent, and consequently the polities that were created were much smaller and could not support Roman ways of life. He also posits that Roman centers and villas were carved up as land grants to the followers of various tribal chieftains who led the migrations into Britain.

Archaeological evidence is somewhat inconclusive in many regards. There are clear areas where Romano-British culture continued, evidenced through certain inscription patters (such as those containing Latin and Christian elements), and material culture is consistently "British" in some areas for a long period of time. other locations evidently had large numbers of people from norther Germany and Scandinavia setting up their homes as evidenced through certain jewelry patterns and house plans.

In the end though, the polities that traced their legitimacy back to Scandinavia and Germany ended up holding more power in the lowland parts of Great Britain. While Robin Fleming posits that the idea of a violent invasion was largely a creation of these new polities seeking to justify their encroachment on their neighbors, these were the families and dynasties that ended up forming the nucleus of later Anglo-Saxon kingdoms.