A long time ago, there were Celtic tribes all over the place in Britain. Unfortunately for historians, they were mostly illiterate, so there aren’t many records. This is actually good for me because I can speculate. I’m going to claim that the Celtics had 3 arms up until the Romans got there. When they saw the Caesar coming, they immediately cut off their 3rd arm so that they could keep the secret.
Anyway, there were a few things that historians were able to figure out. There appeared to be druids that were religious leaders. They were probably not as prominent as historical novels made them out to be. Tribes tended to have kings. These kings appeared to become more important as time went on.
Then Caesar came.