So what has changed exactly? My 1991 edition of “The Oxford History of Britain” discusses the Claudian invasion of 43AD and reads....
“The invasion met with fierce resistance from some of the British tribes. Others, no doubt not sorry to see the Catuvellaunian hegemony in southern Britain destroyed, surrendered easily or joined the Romans.”
....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.