"In Britain, slavery continued to be practised following the fall of Rome, and sections of Hywel the Good's laws dealt with slaves in medieval Wales. The trade particularly picked up after the Viking invasions, with major markets at Chester[90] and Bristol[91] supplied by Danish, Mercian, and Welsh raiding of one another's borderlands. At the time of the Domesday Book, nearly 10% of the English population were slaves.[92]"
https://en.wikipedia.org/wiki/Slavery#History