This is the story of late antique Britain. How in the 3rd to 5th centuries, Britain went through two waves of economic dislocation and transformation, that changed the face of British society.
'Classical' Roman Britain in the 2nd Century
The 'classical' Roman economy in Britain was dominated by the rather remarkable number of soldiers stationed in this remote corner of the great Empire - 40,000 fighting men, in the great legionary fortresses at Caerleon and York. It meant an economy of the size not be seen again for 1,500 years or more. But it was a curious overlay on native British society - hardly touching them. Foreign troops, foreign supply agents, foreign cities, coloniae, established by army veterans. And of course foreign tax collectors. In this world there were big, 'public' cities where bureaucracy happened, where traders worked, where old soldiers lived. The main Coloniae were Colchester in the east, London, Lincoln and York.
But some regional organisation was needed - to adminster the empire, to collect tax in kind. And so were established the regions - the civitas, and each civitas had a capital; places by Gloucester and Wroxeter in the west, Chester in the North east, Leicester in the midlands and so on. Small towns were non existent or irrelevant. (map from Wikipedia).
Change - the 3rd century
Then in the third century, the Empire faced a series of challenges and external pressures, and
suddenly Britannia looked liked either a relatively peaceful place, or a rather unimportant place depending on your viewpoint. And so soldiers left Britannia to do other things - and behind it, the economy collapsed. Or rather it changed. Local industries grew up to replace the old import driven approach; a low value, money based economy. Actually now Britannia's peoples were fully integrated into the economy - producing and consuming, adopting Roman lifestyles - without the sun. Massive villas grew up, owned by locals. Small towns appeared. The economy was smaller - but much wider.
Calamity - the 4th century
But between 290 and 360 this new economy and society was shattered by a series of hammer blows and barbarian raids. And by 410, the money economy had gone, the Roman soldiers had gone. The villas were abandoned, for the most part towns deserted. The British struggled to remodel their lives as best they could. In some places, such as Cadbury, the old Iron hillforts were re-occupied. In others, and local strongman, or Tyranni, established petty kingdoms - like at Wroxeter. And in other places for a while they held on to the old ways for as long as they could - like Birdoswald on Hadrian's Wall. But the old prosperity, social hierarchies and comforts were gone.