@Allamanda It wasn't drift but a pretty complete switch. Why would the Celtic languages become more fringe geographically than before just because the Romans withdrew?

I tend, very often, to think when I look through British/English 'History' that the whole story has been written to illicit a kind of surge of sympathy from the reader based upon something akin to " Oh, those people, invaded and conquered ever so many times, so much were they oppressed and persecuted by those nasty, terrible, war-mongering WE MUST have very deep sympathy and understandings for them."
It is kind of like the 'alleged Persecutions' by the Ancient Romans that the Xrstians so tediously and regularly whine about, persecutions that costs thousands and thousands of Xrstian lives, those self-same persecutions that, Historically NEVER really occurred any way since Ancient Rome NEVER sought to crush out the beliefs of those it conquered, they did give a frog's fart IF you worshipped a bucket full of horse-shit or anything else for that matter, ALL that mattered to the Romans was that you gave your allegiance to the Emperor and the Empire and paid your taxes when required.
It has been proven ever so many times historically that the Romans would adopt and adapt the beliefs of others INTO their own religious beliefs, the Religion of Mithras is ONE prime example, it was very wide-spread throughout Asia Minor, the Romans conquered the areas and then adopted Mithras into their Pantheon of Gods and Goddesses just as they did with the Pantheon of Ancient Greek Gods and Goddesses as well as the many of the Celtic ones as well.
So, as they say on a certain television show, " Myth Busted" for the Xrstian Persecutions myths.

Yes indeed. I laughed when I first read your comment before realising it's not funny.