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I guess you (the way you're interrpreting what Maciamo said) are making a quite common mistake, which is assigning a specific haplogroup to a specific language or language family. Yes, there may be a particularly strong association between a particular subclade of a Y-DNA haplogroup and a certain language group, but it doesn't mean the contrary statement, which is: if a certain people shifted to another language they will necessarily shift their Y-DNA makeup, too. That's especially not true if the languages in contact are already very close and easily learned by each other, as certainly the British languages derived from Proto-Celtic or Pre-Proto-Celtic were from Hallstatt Q-Celtic, above all, and from La Tène P-Celtic. Those "Para-Celtic" languages would've split just 1,000-1,300 years before the arrival of Q-Celtic, so these two would've been probably as diverged as Portuguese and Spanish, or French and Catalan. It isn't really far-fetched to imagine that these people would shift their language to a much more influential, widespread and high prestige language that is already quite close to theirs, especially if, as it appears, there was at least a bit of migration from the continent to the isles, with the speakers of the "new" prestige language among the more ancient inhabitants. When P-Celtic of Gauls and similar tribes arrived much later, the similarity was even greater, with Q-Celtic and P-Celtic probably being sister languages diverged only in the later 500-1,000 years. It would've been not much different from a gradual linguistic convergence from one language to the other, much like what's happening now with Aragonese and Leonese diluting and disappearing into Castillian.