Certainly, much of the infrastructure created by the Romans never disappeared. For starters, the Roman walled cities of London, Chester, York, Lincoln, Exeter, Dorchester-on-Thames, St Albans, Chichester etc all became very significant towns in the Anglo-Saxon period, and their Roman walls continued to be used and modified all the way through the Middle Ages, with sections of the Roman fortifications still surviving today in all the aforementioned places. This gave Alfred the Great and his successors a significant advantage when they were creating their networks of burhs - planned towns fortified by stone walls, as while some like Oxford had to be founded anew others like Chichester had a full Roman wall circuit with sixty towers. Likewise, many of the Roman forts on the Saxon shore, built in the 290s, were still standing. It was inside the Roman fortress at Pevensey, whose Roman walls still survive, that William the Conqueror, immediately after he had landed in late September 1066, built a castle inside the Roman walls, as is depicted in the Bayeux tapestry. Indeed, there was a strong awareness that a lot of England's towns had had their origins as Roman fortress towns - those with Chester in the name derive from the Old English word caestra which is itself derived from the Latin castrum. A largely intact Roman lighthouse also survives to this day at Dover in Kent, next to which an Anglo-Saxon church was built in the 7th century and then in the 12th century the Norman castle was built around it. Roman road networks survived, and the West Saxon kings saw it as their duty to maintain them i.e. in 10th and 11th century charters granting lands to monasteries, they often exempted the peasants living on those lands of the Geld (the land tax) and various royal services, but never from military service and repairing roads and fortifications.

At the same time, many of the physical remains of Britain's Roman past, such as villas, aqueducts (one is definitely known to have existed near St Albans), amphitheatres (like the ones at Chester and St Albans), bath complexes (like the one at Aquae Sulis/ Bath) etc, do seem to have disappeared in the first two centuries after Roman abandonment of Britain in the early 5th century i.e. the Anglo-Saxon Chronicle claims that the baths at Bath fell into ruins in the 6th century, and many of these would not be rediscovered until modern archaeologists got round to it in the 19th and 20th centuries. How rapid abandonment was is hotly debated by archaeologists nowadays, and a recent excavation of a mosaic dating from c.425 - 450 at Chedworth Roman Villa in Gloucestershire suggests that in some places it was far from immediate, but by the 7th century it seems fairly clear that Britain was a changed place. Then we've also got to factor in how a lot of Roman remains were recycled, with Roman stone and brickwork being used to build early Anglo-Saxon churches. This is very obvious with very early Anglo-Saxon churches like St Peter on the Wall in Essex, built in c.650, but Roman brick quoins can still be found above windows in churches dating from the 11th century, like the one at Fetcham in Surrey.

Further reading you might be interested in

Simon Esmonde Cleary, "The Ending of Roman Britain" (2000)

Bryan Ward Perkins, "The Fall of Rome and the End of Civilisation" (2005)

John Blair, "Building Anglo-Saxon England" (2017)

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