Britain was a backwater between the departure of the Romans and the establishment of the Empire.
Unfortunately, the emerging archaeology is beginning to make that look like the propaganda it is. Very useful propaganda for the nascent Catholic church to justify missionaries and the need to "convert the heathens". For some reason the missionaries surprise at finding Christianity already extant in the British Isles has been lost to posterity. Mind you, so too has the indigenous Christianity - although it was much less patriarchal and more inclusive than the Roman brand, so it's probably a good thing. I mean women ? Preaching ? Well Samuel Johnson had that one nailed
.
The reality is Britain reverted to what it had been pre-Roman. Why would it not ? Romanisation only really benefited an elite few anyway - it made very little difference to the vast majority.
But, irritatingly for people who want to bang on about Britain as if it's somehow a pure, untouched island (have you heard our language for Christs sake ?) the one thing which has been inescapable - in fact essential to life has been the constant trade with the rest of the world. Cornwall was not only a treasure-trove, but a massive port, importing wine, precious metals, exotic foods, and people since before recorded time.
(Must be time for a Stewart Lee clip
)