I saw a map of Caesars Gallic Wars and in the middle of the southern coast of Britain was labeled a tribe called Atrebates. Is it just coincidence that they have the same name as the Belgic Atrebates or were they the same tribe?
They were not the exact same people, but were the same cultural tribe. Commius was king of the Belgic Atrebates and an advisor to Caesar before fleeing to Britain and establishing himself as king of the Atrebates there.
Caesar writes about the strong connections between Gallic and British tribes in his Commentaries on the Gallic Wars. Most British historians; Barry Cunliffe, Graham Webster, Sheppard Frere, accept this conclusion.