....some Britons fought in the Roman ranks.
It's always that way. Some locals side always with the invaders. I would refer to them as Quislings but it is too loaded a word.
No it is wrong to call them traitors. A National identy did not exist for these people. Their loyalty was to family-clan and tribe. The rival tribes were enemies. Allying with the Romans to vanquish a rival tribe was simply and example of “the enemy of my enemy is my friend” to them.