Sadly not much information exists. Early Medieval slavery in England is a little unusual in that we know a great deal about some elements of it as an institution, it was centered on the west coast for example, was stamped out following the Conquest, was deeply tied to the economy and Church of England, but we have little information on who the slaves were. There are no literary sources that deal with who the enslaved people in early Medieval England were (beyond some off hand mentions in Italy). Indeed the scope of slavery in the English economy is a little uncertain as well. In the immediate post-Conquest era, and the compilation of the Domesday book, the population of England that was enslaved is unclear. It is likely that the numbers were relatively high though, perhaps as much as 30% of the population of rural England may have been enslaved.

Now there are some thing that we can use to put together a largely speculative. Given that the major center for the English slave trade was Bristol on the western coast of England, and the prominence of Norse raiders in the slave trade (as procurers and merchants) and their bases in Ireland and around the northern coast of Britain, it seems likely that the major source of slaves was not the pagan lands of far Northern Scandinavia, Russia, or the Islamic lands of North Africa and the Mediterranean world, but Ireland, Wales, and maybe a few other spots around the broader North Sea world. Church, and "secular". condemnation of enslaving Christians were likely little more than just that, words, as slaves were an important part of the operation of England's rural agricultural economy, including the extensive land holdings of the Church.

Now the picture for Scandinavia is likewise murky. The Norse operated as slave traders on a much wider scale than the English and were involved in the slave trade from Iceland in the west to the Abbasid Caliphate in the east, and it is likely that Christians, Pagans, and Muslims all were enslaved by the Norse, but how many went to live in Scandinavia? That's not as clear. Most of the involvement of the slave trade in Scandinavia seems to have been as dealers, capturing and selling slaves for profit around the world, and while there were undoubtedly slaves who were "kept on" by the Norse in Scandinavia, the identities of such people is not clear either. Proximity would indicate that the English, Irish, and other British peoples were an option, as were their own Scandinavian and Baltic neighbors.

Early on the Viking Age there is evidence from England that selling English slaves out of England was at least frowned upon by the law, so it seems that the majority of slaves from Britain would have been captured in raids and not just "legally" bought. They then could be brought to Scandinavia, or supplemented with slaves taken from modern Russia and the Baltics for sale in eastern markets such as Baghdad or Constantinople.