[MargaretR]

The Romans didn't colonise us in quite the way you describe.
They were more like an army of occupation which left.

They established a system of government, sent taxes back to Rome, and established town style living.

They left as the empire declined, in order to defend other parts of the empire.

When they left, the small percentage that stayed were not true Romans, but persons who had been resident in other parts of the empire and who had been granted Roman citizenship due to army service. Most of them had never even been to Rome.
They stayed because they had integrated and married locals.

It always amazes me that people conclude that one small city can produce enough people to invade and rule an empire - they didn't - the term 'roman' applied to many nationalities who happen to have served in the roman army.

Our local fort at Ribchester housed 2 cavalry garrisons who originated in Iberia and, what is now, Hungary.

After the Romans left the system of government broke down and there followed 'the Dark Ages'.