Researchers who study medieval slavery typically assume that about 10-30% of most medieval societies were enslaved. We see them in tax assessments like the Domesday Book, property inventories, and occasionally even in miracle stories and histories. That said, they get short shrift in the general scholarship, and if they appear in popular representations of the middle ages (movies and things), these representations are typically informed by what we know about American slavery before the Civil War. Although there was a lot of variety in the types of slavery or unfreedom practiced in the middle ages, it's doubtful that it ever looked quite like the peculiar institution of the US South.

Regarding England in 1086, estimates rest upon an early survey of the Domesday Book​ by H. C. Hardy, who tallied 28,235 slaves among a rural population of 268,984 (10.5%). These numbers may be interpreted in various ways. One of the big debates is whether slaves are counted individually (like animals) or as heads of households (like other people). Furthermore, slaves weren't recorded at all in one of the six circuits that comprise the Domesday Book, which is probably just a difference in accounting rather than a difference in populations.

I fall in the camp that suspects these slaves were counted as heads of households, like other people. I see two reasons for this. First, that aligns better with the fact that slave status tended to be a slippery slope, which would be harder to explain if they were accounted as animals rather than people. Second, it seems that slaves were not managed individually but were rather expected to return a certain amount of goods every year. For example, in one set of laws from the early 900s, if slaves were caught working on a Sunday, they were punished by royal authorities; their masters were neither expected to enforce this rule nor were they held culpable for their slaves' illicit work. With that in mind, up to 30% of the population of England might have been enslaved, or perhaps 600,000 people out of an estimated total population of about 2 million.

What did these enslaved people do? Most probably worked the land. In a Latin primer from the late 900s known as Ælfric's Colloquy, young monks were taught a Latin version of the kinds of complaints they might hear from an enslaved tenant farmer:

Master, I have to work so very hard. I go out at the crack of dawn to drive the oxen to the field and yoke them to the plough. For not even in the bitter winter would I dare to stay at home for fear of my lord; but, when I have yoked up the oxen and fastened the plough and the ploughshare to the plough, then I must plough a whole field or more for the whole day. ... I have one boy who drives the oxen with a goad. He is hoarse from shouting and the cold. ... Yes, indeed, I do very much more. I have to fill the stable with hay for the oxen, water them and take their dung outside. Alas, I have to endure such hard work since I am not a free man. (full trans. here as pdf)

Additional tasks might have been more diverse. Women were always in demand to be enslaved as domestic workers, textile producers, wet nurses, and sex workers. Anglo-Saxon lawmakers repeatedly prohibited priests from taking concubines, which suggests that they considered this an enduring problem. Some slaves might also have had particular training. Alfred the Great (r. 871–899), according to his biographer, had children of obscure birth (multis ignobilis) educated alongside his son Æthelweard, and he commanded that his judges have, at the very least, a slave who could read for them if they themselves were illiterate.

Slave status was generally inherited, although free individuals could be captured in raiding. There were laws between Wessex and the Danelaw saying that witnesses needed to be present when slaves were traded across the border, ensuring that these people were considered legal slaves and not victims of illegal kidnapping. But there were also longstanding laws against selling slaves outside of England, dating back to the early 700s, which is basically as far back as our legal knowledge goes.

Our knowledge about the decline of slavery in England is more shaky. It seems like slave status simply stopped being meaningful and people began being classified in other ways. Perhaps this had to do with the fact that land was relatively abundant but labor was limited, particularly in the wake of poor harvests or plagues. Perhaps it had to do with landlords simply wanting to streamline their account books. At any rate, enslavement had largely been abandoned in Northern Europe by the early modern period. For England, at least, this seems to be more a process of shifting terminology than abolition and emancipation, which meant that there was no reason early English law couldn't accommodate the later expansion of the Atlantic slave trade.

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Further reading:

• Hardy, H. C. Domesday England. Cambridge: Cambridge University Press, 1977.

• Wyatt, David. Slaves and Warriors in Medieval Britain and Ireland, 800–1200. The Northern World 45. Leiden: Brill, 2009.

• Pelteret, David A.E. Slavery in Early Mediaeval England: From the Reign of Alfred until the Twelfth Century. Studies in Anglo-Saxon History. Woodbridge: Boydell Press, 1995.

• Clunies Ross, Margaret. “Concubinage in Anglo-Saxon England.” Past and Present 108 (1985): 3–34.

• Stuard, Susan Mosher. “Ancillary Evidence for the Decline of Medieval Slavery.” Past and Present 149 (1995): 3–28.

• Gillingham, John. “Women, Children and the Profits of War.” In Gender and Historiography: Studies in the Earlier Middle Ages in Honour of Pauline Stafford, edited by Janet L. Nelson, Susan Reynolds, and Susan M. Johns, 61–74. London: Institute of Historical Research, 2012.

• Karras, Ruth Mazo. “Desire, Descendants, and Dominance: Slavery, the Exchange of Women, and Masculine Power.” In The Work of Work: Servitude, Slavery, and Labor in Medieval England, edited by Allen J. Frantzen and Douglas Moffat, 16–29. Glasgow: Cruithne Press, 1994.

• Ruffing, John. “The Labor Structure of Ælfric’s Colloquy.” In The Work of Work: Servitude, Slavery, and Labor in Medieval England, edited by Allen J. Frantzen and Douglas Moffat, 55–70. Glasgow: Cruithne Press, 1994.