There were movements of people, and movements of culture, which could be separate, but Celts are late Indo-European arrivals to the British Isles, either directly from the Celtic heartland in Germany, or indirectly along the western coasts from Spain.
The Celts just happened to be the culture in place when the first accounts were written by the Greeks and Romans. So they have self-appointed themselves to be poor down-trodden invaded brutalized indigenous people - but they were just the previous wave of ruthless invaders!
There were people in the British Isles 10s and 100s of thousands of years ago. Britain was wiped clean by ice ages and repopulated. Britain was connected to Europe by a land bridge, etc. etc. All this long before the Celts turned up.
https://en.wikipedia.org/wiki/Prehistoric_Britain
There is no Celtic presence in Britain before about 500 BC. Stonehenge and Guadalperal are much, much too old to have been built by the Celts. Celtic druids did not build Stonehenge.
"While the Dolmen of Guadalperal has widely been compared to Stonehenge—and rightly so—the Spanish example was once an entirely enclosed space. And it could also be around 2,000 years older."
kind of dovetails to https://www.independent.co.uk/news/uk/this-britain/celts-des...
" A team from Oxford University has discovered that the Celts, Britain's indigenous people, are descended from a tribe of Iberian fishermen who crossed the Bay of Biscay 6,000 years ago."