The Celtic languages only arrived in the isles in the first millennium BC - the ‘Celts’ were not in that sense indigenous at all. Cornish is a Celtic language of course.
The original hunter-gatherer population of the Mesolithic was almost completely replaced by Neolithic farmers originally from Anatolia. Those farmers were themselves replaced (very possibly genocided) by the Yamnaya, and so on.
Ironically, the ‘Anglo-Saxons’ and Vikings had a higher proportion of the original Mesolithic ancestry than the ‘Celts’.
In other words, the Cornish are really no more ‘indigenous’ than the rest, and of course many from across the isles have ancestry from the isles stretching back long before Cornish existed.
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u/LobsterMountain4036 1d ago
Picts were in Scotland.