“by 400 Irish and British were fully differing languages”?
British is now a language? I assume they’re talking about English, but in their effort to remove the word “English” and to erase the historical record, they are “Britishizing” everything.
Irish (Q-celtic) and British (P-celtic) were differing before, and this had nothing to do with the English. Odd really that the Saxons were the majority of the Anglo-Saxon settlement of Britain (from which they took the name much later) but the country was named after the Angles.