In ancient times slavery was a common feature of life in many parts of the world, including the British Isles. People became slaves through being taken captive in war or by slave-raiding parties, or as punishment for crimes, or they sold their children into slavery or became slaves themselves if they could not afford to pay a fine or a debt.
A trade in slaves existed in the British Isles even before the Roman conquest. Slavery continued after the fall of the Roman Empire and into the medieval period; according to the Domesday of 1086 roughly 10% of the population of England were slaves, although slavery was already in the process of being phased out at that time.
In 1066 the enslavement of Christians was abolished, and over time slavery was replaced by serfdom. Unlike slaves, serfs were tied to the land, and could not be bought or sold unless the land itself was sold.
Reference: Wikipedia: Slavery in the British Isles