The population of roman Britain is estimated at 2-3 million at its peak. Well above the early medieval population.
https://www.britannica.com/place/United-Kingdom/Roman-Britai...
https://books.google.se/books?id=t7KeBAAAQBAJ&pg=PT260&redir...
https://www.britannica.com/place/United-Kingdom/Roman-Britai...
https://books.google.se/books?id=t7KeBAAAQBAJ&pg=PT260&redir...
Direction of changes, short term, are more reliable where there is information, but with long periods without information, so only rare baselines.