Are those "just" serfs,

This is the problem with constantly changing historical terminology in pop history. The term 'serf' is commonly used in pop history to denote tenant farmers and peasants who were not freeholders, but the servus in Domesday Book were indeed slaves. While tenant farmers weren't "Freemen", that didn't mean they weren't free men; Free in the context of Freemen or Sokemen instead meant that they were free of service rents.

Slaves in England were predominantly Welsh, but not exclusively. Some slaves were Irish, and penal slavery was also a punishment with changing restrictions based on time and kingdom. The seventh century laws of Hlothhere of Kent, for example, held that a thief caught in the act could be sold into slavery overseas, while the laws of Ine of Wessex actively forbade the selling of English slaves to foreigners. Slavery was covered by a series of legal restrictions and protections; slaves couldn't work on a Sunday, and children born from a liaison between slave and master were freed, for example. A number of laws covered circumstances under which manumission was expected.

Slaves were likely predominantly agricultural labourers, judging by the presence of slave plough teams in Domesday and a slave ploughman in Ælfric's Colloquy. This wasn't always the case, however: a number of wills deal with the inheritance or manumission of slaves which show that female slaves could be employed as spinners, weavers or dress-makers, and could be quite talented.