So, the early Welsh built Stonehenge.
The other question is why did they go to that spot so far away and with such trouble? Why didn’t they just build it in their area?
Before the Anglo-Saxons came they were just Britons. “Wealsc” was the old German word for foreigner, and they drove them into what is now Wales. Of course the Britons of 5,000 years ago predated the Celtic Britons, who displaced them, only to be displaced themselves millennia later.
It’s interesting how many names of peoples we use were given them by their neighbors/enemies. Early Saxons would have identified themselves as member of different Germanic tribes. But they all had the tradition of wear seax knives to show they were free men who were allowed to carry weapons. Their neighbors called them seax-men or Saxons. Likewise the old German name for spear was “ger” or “gar”, so the Romans called them Germans, or spear-men.
I understand that Stonehenge is built at one of the very few places in England where they could build a structure to measure the sun and moon phases and keep that unique design. Even a few miles either way would have thrown it off.