Tin

My ancestors (those wot ain't Jewish or Asian) are mostly Celts, one way or another. Even the "Saxons" and "Normans" in my family tree were highly Celt by the time they took ship. The first of us arrived in the British Isles with "bell beaker" pottery. That would be 2500 BC. This is considered "Bronze Age".

There are lots of ways to make "bronze". "Bronze" just means "copper alloy". When such alloys were first mooted they were copper and ... arsenic. Later they switched to alloying with less-toxic tin. By the mid 2000s BC surely.

Tin isn't so easy to get, as is arsenic. The Bronze Age warriors had to search to the ends of the Earth for it. It happens that, historically, tin was rich in that peninsula we're now calling "Cornwall".

One drive which encouraged the first Celts to invade and to conquer Britain must have been the tin deposits in, exactly, its West Country.