The answer is easy: Tin.
Tin, to make bronze, was hard to obtain. Wales and Corwall are sources of tin from ancient times. Traders from the Med came to Wales (and Cornwall) to obtain tin and left their genetic legacy there.
We have a winner!!! Sorry I was tardy, I was out of town. But Tin is the answer. Tin was very rare in the ancient world and highly sought after. Archaeologist are still searching for the early sources of tin used in the Early and Middle Bronze age because they think that Cornwall and Wales are just to far away. Actually tin is not really used purposely (so we think) until about 2000 BC. Bronze made before that is alloyed with arsenic or the tin is a bi-product of the copper.
Think of this, the article says that the genetics go back to 2000 BC, the start of the pre-palatial Minoan period, and tin is mined at the same time. Interesting that the Minoans or some other people may have went as far as Wales for tin.