That is also bullshit (although a popular myth, deliberately cultivated by 19th-century English supremacists to justify subjugating their Celtic neighbors). Every region in Britain has a majority of Insular Celtic DNA on average (though the fraction varies). Even in the most heavily Anglo-Saxon regions like East Anglia, Germanic DNA is in the minority.

https://www.ox.ac.uk/news/2015-03-19-who-do-you-think-you-re...

And of course the Celtic Britons replaced the Neolithic farmers, who replaced the Mesolithic foragers, and so on. "Indigenous" becomes meaningless with so many population replacements.