So while the first literary references to King Arthur do come from the 1100s, in Geoffrey of Monmouth's Historia Regum Britanniae, "Arthurian Britain" is set in the immediate aftermath of the Roman withdrawal from Britain and the arrival of the Anglo-Saxons in the 5th Century. Sources are extremely lacking for this early period, limited largely to Gildas and, to some extent, Procopius. The best we can say is that there was probably slavery, as a hang-over of Roman Imperial rule.

In the intervening period, slavery definitely existed and was an important part of Anglo-Saxon society. I've written about slavery in Anglo-Saxon England here. The majority of these slaves are likely to have been Welsh or Irish, at least by the ninth and tenth centuries.

As I mention in that earlier answer, slavery appears prominent still in the wake of the Norman Conquest, with 10% of 'households' in Domesday (1086) listed as slaves. By the 1100s, however, slavery appears to have disappeared, likely due to changing social views post-Conquest and pressure from the Church. Debate rages over whether a slave 'household' represents multiple occupancy as with other households - in which case there was major social reform - or just individual slaves - in which case that 10% of the population reduces to around 2-2.5% and explains the rapid decline much more easily.

E: From what references to slavery in legal codexes and Domesday Book tell us, slavery appears to have been largely used as a resource for agricultural labour, rather than necessarily domestic service. Certainly slavery does not appear to have been organised on the mass scale used by the Romans for large construction, as the maintenance of infrastructure was a burden exacted through taxation, and the Anglo-Saxon Chronicle notes that the army or fyrd were often used in the construction or maintenance of fortresses.