Hello, I wrote an answer here about Romanitas - the pursuit of a perceived Roman socio-political-cultural ideal - and how it was central to early medieval politics. In Anglo-Saxon histories, the Romans serve both as aspirational role-models and as a moral caution to the English. Poems such as The Ruin from The Exeter Book, for example, discuss ruefully the crumbling physical relics of Roman occupation and regret that such construction is beyond the capability of the early English to the extent that it may as well have been done by giants. This is not to say that the technolgy was lost, but rather that isolated Anglo-Saxon successor kingdoms lacked the sheer logistical and infrastructural scale of the Roman Imperial machine which made such vast construction possible. In the 10th Century, as Æthelflæd of Mercia restored the Roman city of Chester from ruins, she minted a series of celebrational coins, themselves inspired by Roman triumphal designs, to celebrate the fact that a resurgent Mercia was now able to match, and indeed restore, the glories of the Roman past.

The English are quite clear about the decline of the Romans and why things 'regressed' in Britain: according to Bede, Britannia was "hurriedly stripped of all troops and military equipment and robbed of the flower of its young men" by "ambitious despots" as the over-reaching Roman Empire fell victim to greed, treachery and descent into a series of coups and civil wars. Shorn of its military capabilities, the Romano-British authorities now possessed "no engineers capable of such an undertaking" of maintaining the series of Roman walls and fortresses protecting Britain from Irish and Pictish raiding.

The Romano-British successor state eventually stabilises and even flourishes, but is eventually laid low by its moral turpitude, falling victim to the same failings as its Imperial predecessor: "with plenty came an increase in luxury, followed by every kind of crime, especially cruelty, hatred of truth and love of falsehood... the Lord's flock and their pastors giving themselves up to drunkenness, hatred, quarrels and violence." Within Bede's Christian moral narrative, God's punishment is swift and severe: first, a "terrible plague...destroyed so large a number that the living could not bury the dead" followed swiftly by the arrival of the Anglo-Saxons, who are portrayed as a manifestation of divine wrath, summoned forth to lay low the Britons for their wickedness.

If we forgo Bede's moralising, there's no mistaking a post-Roman decline in Britannia, brought about largely by the collapse of the Roman Imperial economy and infrastructure and the resulting provincialisation and logistical collapse this brought, denuding the British Isles of their military capabilties and severely limiting the capabilities of provincial authorities to even attempt to maintain the status quo. The eventual arrival of the Angles, Saxons and Jutes and the slow coalition of English kingdoms brings the gradual return of stability and prosperity that eventually allow for something approaching the Roman model to be re-established, and its important to note that Classical knowledge remained relevant throughout the period. Medical textbooks such as Bald's Leechbook drew as much on Greco-Roman medical texts as they did on tried and tested Germanic and Brythonic "folk" medicine and herbal remedies.