John Morris’s magisterial history, The Age of Arthur, is full of astonishing insights into the transition between Roman Britain and Anglo-Saxon England, none more so than his description of the role played by the great plague of Justinian in the middle of the sixth century. This plague is estimated to have halved the population of civilised Europe and is the first recorded epidemic definitely attributable to Y pestis. It may have originated in Egypt or Ethiopia and been carried to Constantinople by fleas on the rats which thrived on grain ships. From 541 it spread gradually northwards across the old Roman provinces, reaching Britain around the end of the decade.
In 550, the island of Britain was predominantly British, i.e. a Welsh-speaking land of warring Celtic princes with bands of horsemen who ate up the produce of subsistence farmers. They were unable to maintain an urban civilisation after the departure of the Romans in 410, but were successful at keeping the Angles and Saxons (i.e. English) confined to Anglia and Kent. There was no trade or social exchange between the Christian British and the pagan Angles and Saxons, once they had had fought each other to a standstill under King Arthur. The British carried on some trade with the Mediterranean, whereas the English lived on what they could grow. So when the plague reached Britain in boats from mainland Europe, it killed up to half of the native British population but left the English colonists largely unscathed.
Not long afterwards, the English began to mount probing raids into British territory and found that there was little opposition. They sent word back to their relatives in Schleswig-Holstein and the Danish peninsula that the whole island was up for grabs. The king of the Angles was so impressed that he put his entire population into boats and left the area west of Hamburg deserted for several centuries. And so, 150 years after Hengest and Horsa first brought in Saxon warriors to police the borders of crumbling Roman Britain, the English decisively colonised plague-ravaged Britain from the borders of Wales to the middle of Scotland.