Evidence I think shows was plenty of back and forth trade and population movement between Britons and Gauls in that period. And at this point in history or just prior I suspect Brittonic and Gaulish were more dialects or branches of a common P-Celtic language, but there is controversy about this topic.
In any case, language aside, esp after the Romans conquered Gaul, many Gaulish tribes had power centres among the Britons in areas of what is now England. There's been chariot burials discovered in Britain that look basically identical to those from the continent.
E.g. the Parisi were a Brittonic tribe mentioned by the Romans that share a name with the Parisii of Gaul (for which Paris is named), and likely were quite connected.
I believe in fact that part of the justification the Romans gave for their invasion of Britain was in fact that the (defeated) Gauls were using it as a power base to cause problems for the Romans in Gaul.
And then a few hundred years later, it got flipped around and some Britons fled from Saxon invasion back to the continent into what is now Brittany in France and Britonia in Galicia in Spain.
Brittonic was likely very close to gaulish. It's possible the authors just had a simple mixup or possibly subscribe to one of the various hypotheses that they're genetically closely related (e.g. gallo-brittonic).
In any case, language aside, esp after the Romans conquered Gaul, many Gaulish tribes had power centres among the Britons in areas of what is now England. There's been chariot burials discovered in Britain that look basically identical to those from the continent.
E.g. the Parisi were a Brittonic tribe mentioned by the Romans that share a name with the Parisii of Gaul (for which Paris is named), and likely were quite connected.
I believe in fact that part of the justification the Romans gave for their invasion of Britain was in fact that the (defeated) Gauls were using it as a power base to cause problems for the Romans in Gaul.
And then a few hundred years later, it got flipped around and some Britons fled from Saxon invasion back to the continent into what is now Brittany in France and Britonia in Galicia in Spain.
This is a good book on this topic: https://www.amazon.ca/Blood-Celts-New-Ancestral-Story/dp/050...