[vraid]

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...

[ncmncm]

Historical population estimates necessarily have a very large admixture of conjecture.

Direction of changes, short term, are more reliable where there is information, but with long periods without information, so only rare baselines.

[NeedMoreTea]

Thanks. Have to say I'm surprised, think I have a new period of history to read up on for 2020!