[muawiyah]

You are jumping right past the Dark Ages. When the Bretons first began moving from Brittania to Brittany they reported that the entire countryside was devoid of people, and that even the animals had disappeared for the most part.

The tradition is that Merlin had to replant the vinyards in Beaujolais!

BTW, way back then in the 7th and 8th Centuries, Brittany was considerably larger than today.

The German people were still fleeing the Continent at the time ~ see Angles, Saxons, etc.

So, you can name all the Continental Celtic tribes you want ~ they were, for the most part, reduced to a handful of people by whatever brought about the Dark Ages.