Here is the amazing fact: today, 16 of France’s 20 largest cities are located on or near a Roman town, while only 2 of Britain’s 20 largest are. This difference existed even back in the Middle Ages. So who cares? Well, Britain’s cities in the middle ages are two and a half times more likely to have coastal access than France’s cities, so that in 1700, when sea trade was hugely important, 56% of urban French lived in towns with sea access while 87% of urban Brits did. This is even though, in both countries, cities with sea access grew faster and huge sums of money were put into building artificial canals. Even at a very local level, the France/Britain distinction holds: when Roman cities were within 25km of the ocean or a navigable river, they tended not to move in France, while in Britain they tended to reappear nearer to the water. The fundamental factor for the shift in both places was that developments in shipbuilding in the early middle ages made the sea much more suitable for trade and military transport than the famous Roman Roads which previously played that role.
These days, the French model is looking somewhat better, as Toulouse has held its ground more readily than has Liverpool.