As I responded in a parallel post, it's most likely that Geoffrey of Monmouth based his Uther Pendragon on Eudon Penteur (c999-1079), a contender for the ducal throne of Brittany, an elder cousin of King Edward the Confessor, and the father of many Breton lords who settled in England following the Conquest, including two of great prominence, Alan Rufus and Brian of Brittany. The Bayeux Tapestry, Geffrei Gaimar and Wace of Jersey all attribute much of Duke William's victory at Hastings to Alan's efforts.
Alan, as the leader of the Bretons in England, was one of the three witnesses to the foundation charters for the Lordship and the Priory at Monmouth. The other two were the new lord Withenoc de la Boussac and Withenoc's brother Baderon.
Geoffrey drew the names Uther and Arthur, as he stated, from a Breton source, in my view the 9th century Cartulary of Redon, wherein they appear as charter witnesses.
The names and places of origin of "King Arthur's family" run parallel to those of members of Alan Rufus's family: his father Eudon Penteur, his mother Orguen of Cornouaille, his mother's brother Count Hoel, and even his beloved Gunhild of Wessex parallels Guinevere of the West Country.
Alan's paternal uncle Duke Alan III was poisoned to death, so Geoffrey gives the same fate to Arthur's paternal uncle Ambrosius Aurelianus.
Alan Rufus's epitaph describes him as a star, shining orange-red, and emphasises its brilliance. Well, the brightest star in the northern celestial hemisphere is Arcturus.
As Arcturus is the guardian of the Two Bears, Ursa Major and Ursa Minor, so Alan Rufus was the captain of the household knights of Kings William I (William Major) and William II (William Minor).
Due south of Arcturus is the constellation Virgo, representing in medieval thought the Virgin Mary. Brittany's symbol, ermine, was borne on the shields of Alan's men, and it represents honour and the Virgin Mary.