When Romans Left Britain, What Happened?!?

Most people think of Great Britain as THE colonizing force of history. The sun didn’t set on the British empire for centuries, and here we are all these years later speaking English.
But, what most people don’t know is that Britain itself was once colonized. Britain was conquered by Rome starting in AD 43, and was a province of the Roman Empire for hundreds of years – or about 20% of its recorded history.
But then, in the fifth century AD, the Roman legions withdrew from the British Isles. The colonizers left, returning Britain to the indigenous inhabitants.
So, now that the people of Britain finally cast off the yoke of colonialism, freedom and prosperity followed, right?
Not exactly.
Less than 50 years after Rome withdrew, civilization in Britain was crumbling. Cities as well as rural communities were abandoned. Society and the economy were devastated. London may have been vacated during this time. In short, things fell apart. But why?
Well, the departure of the Romans left Britain vulnerable to NEW invaders, namely Germanic tribes like the Angles, Saxons, and Jutes. These were illiterate, pre-Christian peoples, and no doubt they helped to topple all that Rome had built in Britain.
But most of these invasions happened long after Roman legions withdrew. And evidence suggests that the collapse of Britain began before the Germanic tribes arrived.
In short, what Rome had built in Britain was too advanced, too complex, and too fragile for the indigenous people to sustain without Rome there to help them.
A Forgotten Backwater
By the end of the 6th century, Britain was a forgotten backwater of Europe, an island of illiterate pagans. The population dwindled. Markets vanished. The booming economy of Roman times regressed into scattered subsistence farms. The use of coins declined. The mass production of pottery ceased. Roads fell into ruin. Central heating and hot baths disappeared for hundreds of years.
Bricks, which the Romans used for construction, wouldn’t reappear in Britain for another 900 years! Glass-blowing wouldn’t return to Britain for 1,200 years!
Militarily, post-Roman Britain was feeble, open to attacks by various raiders and invaders.
Christianity had been widespread in Late Roman Britain, but after Rome left, the Celts gradually returned to paganism. It took centuries of religious missionary work to fold England back into the Christian traditions of the west.

source