The Romans began to leave Britain in 410 A.D as their grand empire began to shrink. Across mainland Europe barbarian tribes were causing all sorts of migrations as Huns, Goths and other tribes ventured into the western areas of the continent - lands rich with over 400 years of Roman administration.
Neighboring tribes that had previously been driven back by the Roman army began immigrating to the British Isles en masse. All of Britain, including Caledonia, to the north, were under threat. The Picts who had been able to keep Rome at bay began to attract interest from Hibernia (Ireland). A northern tribe known as Scoti began to migrate to their northern lands.
In the Romanised parts of Britain's south east; the Germanic people of Northern Europe, known as Saxons, Angles and Jutes began to migrate onto the island. They were fierce tribes fighting to secure lands in southern Britain.
The Roman Celts termed the Anglo-Saxon tribes as invaders. They were pushed back westwards towards Wales, Devon and Cornwall. The stories of King Arthur are about his Britons fighting against Angles and Saxon invaders. Though the legends have become corrupted over time. (Some making King Arthur a King of England when it was the English tribes he fought against.) For the Roman Celts, the demise of their civilization must have been along an apocalyptic scale as they saw their industry and commerce decline and their lands swamped by the English tribes. They had to integrate or flee west.
Eventually the Angles and Saxons became the dominant force through out the entire Isle with their language, one day, becoming most widely spoken. Of all the tribes in Britain, during the beginning of the Dark Ages; the English ones were illiterate. It is strange and quite a feat that their language would one day progress so far.
Of course this language as developed over the centuries and if anyone went back among the Dark Age Anglo-Saxons, I think Dutch people might have a better chance of understanding what they were saying, then people of English speaking nations would.