The Roman era in Britain may be characterized by the country houses, with central heating and hot baths, of the wealthy landowners, protected more by the Roman culture of law than by the soldiers who could be summoned in times of emergency. This lifestyle is what Britain, and other areas governed by Rome, had to lose when the Roman Empire fell. And Britain did lose this when Rome failed, c. 400-550 AD.
But most of Europe didn't. Growth plateaued but did not become decline until an economic crisis, c. 800 AD, led to both famine and reforestation of abandoned farmlands, this event being well decoupled from the 'fall' of Rome. Arguably, the darkest thing about the Dark Ages is the zeal with which the monks and clerics destroyed any written document that did not affirm the latest rulings from Rome- the Rome of the Pope, busily engaged in 'catholicizing' the writings and laws of the church.
Even impoverished Britain, in those chilly days, could afford a few ermine furs, but Normandy, from the late 9th century on, was engaged in building cathedrals, universities, and the cadres of educated men to staff them. They were constrained by their technology, but they were not poor people. Northern Italy was prosperous, with walled cities fed by well-ruled hinterlands, financing and carrying on trade with Constantinople and the Levant. The Hanse had emerged as a loose coalition of prosperous and vigorous cities reaching out to expand their markets and trade. In general, prosperity, here defined as adding 1-2% each year to the economy instead of losing that amount, reigned.
At that point in European history, the nation-state lay in the past- they had tried that, and it failed. They had replaced it with feudalism, a complex system in which each person owed allegiance to, and was governed by, many different jurisdictions and levels, many of which could, and did, conflict with each other. Maybe we can see some of the force of feudalism when we contrast how the nation-states of the past had been gobbled like popcorn by an expansionist Rome, while feudalism knit the entire continent of Europe into a semi-cohesive whole.
The city-state, on the other hand, prospered. It proved unnecessary for the city to legally rule the hinterlands- the city did that naturally by controlling the markets- and cities became compact and powerful centers of productivity, petitioning for charters and special privileges. Ruling themselves feudally, as did their society, they also became the first pillars of self-government.