I would take issue with part of your point regarding the production (and by implication distribution) of Roman pottery in Britain.
While granted that the Gaulish (and other continental) centres of production did have some influence on Britain, as can be seen from the individual pottery distribution maps available at this link to Potsherd website, Roman style 'Fineware' pottery production seems in a few instances to have continued in Britain until around the mid 5th century - long after direct continental imports had ceased.
Unfortunately what the maps cannot show is the extent to which pottery production continued although as you implied it probably was a shrinking market as Roman influenced financing and consequently infrastructure supporting it declined.
And demographics played a part in it as well. The collapse of Roman Britain also seems to have apparently happened in a time of depopulation. It seems like everything was hitting them all at once...invasions by barbarians, collapse of the monetary systems, plagues, internal strife and political instability, ruralization of the surviving populace, interruptions in the economy and food supply...and on it goes. The more we dig the worse it gets. No wonder the pottery industry took a hit.
And again, the funerary practices had been changing since Christianization, so less need for funerary pieces like burial urns, etc as fewer cremations were taking place.
One thing I have seen here is talk about the semi-fine porcelin mentioned here and there, as being something almost like China. Was it China?
Europe didnt have the good Kaolin clay to make China with, but later Europeans could make severe glaze ceramics. So, is there a school of thought that ceramics of the highest order might have been imported from Asia along the silk road?