Theory that fairies are scared of metal due to the metal wielding Beaker invasion crushing the Neolithics.
The theory above in a tweet by Miles @mgdever is interesting but probably wrong.
The Bell Beaker culture brought copper and bronze to Britons in the stone age, but the legendary aversion of fairies to metal is specifically an aversion to iron, with fairies sometimes wielding Bronze weapons.
The British Iron Age, however, started around 800 BCE, which is at a point when the Bronze Age culture brought by the Bell Beaker movement has collapsed and was being replaced by Celtic culture, although probably not with much population replacement.
The relatively shallow time depth of the Celtic languages supports the case for a language shift in Western Europe to the Celtic languages with the associated culture in the Iron Age, even though we know that there wasn't much population genetic replacement at that time. Proto-Celtic is often associated with the Urnfield archaeological culture. This implies that the original language of the Bell Beaker people has been lost except as a substrate in the Celtic language, and possibly in the Basque language (one of several theories).