That's the Galician story. The movement of the Milesians to Brittany took place in the aftermath of the climate anomaly in roughly 535AD ~ they took their Arthurian stories with them which included the chronicles of the British kings ~ in pre Anglo-Saxon times.
Ergo, the Celtic population in Brittany circa 7th Century-9th Century (and so on up to today) was essentially the same as the Celtic population in Ireland, Wales, Britain and Cornwall from roughly 700BC up until modern times ~ there is a short brief at http://en.wikipedia.org/wiki/Kingdom_of_Castile which you might want to review to pick up on the words.
here is a basic differentiation in the two main threads called Goedelic and Brethonic to deal with, and if you want to start reading old Breton language texts, they had about 7 standard ways of spelling so you need to get over that.
Now, your alternate history is the Celts in Brittany are DIFFERENT FROM the Celts in Galicia?
'you'll need to 'splain that one!
“Now, your alternate history is the Celts in Brittany are DIFFERENT FROM the Celts in Galicia?”
So you’re claiming Brythonic and Continental Celtic languages are the same?
I have no doubt that most ancient Europeans shared the same DNA, but that doesn’t mean they all shared the same language.