Robin Fleming makes some interesting observations on this topic in her book Britain After Rome to in relation to Roman Britain's economy being dominated by supporting the over-sized legionary presence required here:

"The army stationed in northern Britain was very large, more than 40,000 men at its height, or about an eighth of the imperial army. This level of garrisoning required the expenditure of something on the order of a sixteenth of the total imperial budget each year, although Britain itself was only one of forty Roman provinces. An army of this size required not only immense outlays of state funding, but constant provisioning with a wide range of goods - food (in particular grain), clothing, arms, transport animals, building materials some supplied by local communities, but much brought from further afield. During the height of the principate, the army contracted supply agents and transporters from provinces across the Channel to provision it with much of the food and matériel it needed”.

She goes on to describe how Britain’s economy was, over the 400 years of Roman occupation, disproportionately shaped by a trade network designed only to supply the provisions required by this huge legionary presence, and the goods that went the other way (e.g. Cornish tin and Welsh gold). Once the legions were withdrawn to fight on the continental mainland, this trade network evaporated more or less overnight. In that context, what society could survive the sudden collapse of an economic system that had been in place for four centuries?

During the four centuries while it had been intact, Roman Britain had comprised Romano-British urban centres (like London, Colchester and St Albans), smaller garrison towns and a network of Romanised country villa estates, just like anywhere else in the empire. Then, alongside these ‘civilised’ Romanised peoples was the ‘barbarian’ peoples who resided outside the civitates. Again, like elsewhere in the empire, these may have been subdued and, to a greater or lesser extent, integrated into the Roman economy via trade.

Once the plug was pulled from this system, the legions and the trade network to support them evaporated, the cities that only existed to support that trade network became redundant and were abandoned and a new patchwork of non-integrated agrarian tribes began to struggle to assert themselves to fill the political void. Some of these tribal groups tried to continue in the Roman way. Ambrosius Aurelianus was the official representative of Roman Emperor Honorius, even after the Legions left, as the empire attempted to retain links with its former colony. He fought against the Anglo-Saxons in the 5th century, still under the banner of some form of Roman-style continuation. Pockets of British Christianity survived too, despite the incoming Saxon (and then Viking) pagans.

But the upheaval of the collapse of Roman life in Britain was so profound, it would take generations for the patchwork of Anglo-Saxon and Welsh tribes to cohere into organised, hierarchical societies with all the hallmarks of settled and functioning economies (unification under powerful kings, international trade, literacy, minting of coins etc.).

Any society, for which a centuries old economic model collapsed suddenly, would find itself in an anarchic power-vacuum, which would potentially take decades or even centuries to shake itself together into a new functioning replacement economy. It takes a planned economy to quarry stone, stonemasons to shape it into blocks, clay pits to be dug and kilns to fire them into tiles. In the absence of that planned economy, people revert to vernacular ways of building houses, where you clear a patch of woodland, fell a few large trees for your structure and build a thatched roof over it for shelter and some pens for your animals. That’s enough for a subsistence lifestyle for you and your family. Only when you have coherent villages and towns capable of generating a surplus can you begin to contemplate the loftier aims of civilisation, which only began to emerge as the scattered tribes began to coalesce into powerful kingdoms.