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After the collapse of Roman rule in the first decade of the fifth century, Britain was divided into a number of kingdoms as shown on this map: http://www.historyfiles.co.uk/FeaturesBritain/BritishMapAD400#Map .

One of these kings was called Vortigern and he became overlord in AD 425 according to one source. He or Vortigern II invited Saxons from Germania to settle in Britain as foedarati. In circa AD 450 they revolted and killed many of the British leaders. Kent was quickly lost to the Germanic invaders and by AD 700 the Anglo-Saxons ruled all of England except for Cornwall.

If the Saxon revolt had not happened or if it had been suppressed, and if successive British high kings were able to unite their people in successful resistance to Germanic invaders in the fifth and sixth centuries, the historical and cultural consequences of an independent Pridean would have been significantly different from OTL.

An independent Pridean might or might not have been able to resist Viking invasions in the eight, ninth and ten centuries. There might have been Viking conquest of the east and north, perhaps even of the whole country. However would have been no Harold Godwinson, so the events which gave William, Duke of Normandy a pretext to invade England in 1066 do not happen.

With no Anglo-Saxon conquest the language of Pridean would have been Brythonic and Old English would have developed differently than in OTL.

Pelaganism was influential in Britain in the first half of the fifth century, so in an independent Pridean a Pelagian Church could have developed, which did not owe allegiance to Rome. If such a Church survived to the reign of Charlemagne, then Pope Adrian I or Pope Leo III could have asked him to invade Pridean to bring it back to the Catholic communion. There would have been the consequences of the failure or partial or complete success of such an invasion.
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