Sub-Roman Britain
I like how this series employs elements of the magical with a modicum of the historical. For example, the first episode takes place in a post-Roman Britain. Uther and his soldiers hold to Roman ways as did the true Brits, for the 100 years following the Roman departure (410 AD). "The Mists of Avalon" also place the events within the context of a sub-Roman Britain.
I've read that a historic Arthur would have lived during sub-Roman Britain and likely fought in the campaigns against the Saxons. He was most likely NOT a king but rather a general of great prowess. His legend owes its roots from Geoffrey of Monmouth's [i]Historia Regum Britanniae[i].