From the End of My Last Post (Why Illtud is Probably Not Arthur's Real Father)

Alternately, it could all be THIS SIMPLE:

Illtud's title is used as the name of an elegy in the Book of Taliesin.  In that poem, the warrior is compared with Arthur (much as another warrior is compared with Arthur in The Gododdin). 

Geoffrey or his source uses this title (as he did the gorlassar descriptor applied to Uther to conjure Gorlois) as Arthur's father's name.  It is likely that Geoffrey did not know Arthur = Ceredig son of Cunedda.  He lacked a record of Arthur's father's name. Furthermore, he may not have known Uther Pendragon was merely a Welsh rendering of Latin titles used for Illtud.  Uther at some point obviously took on an independent existence.

We don't know how Arthur was originally employed as a designation for Ceredig.  I've made a good case for the arth- or bear component found in the royal pedigree of Ceredigion as coming from the Afon Arth. Arthur as it stands is from Latin/Roman Artorius, but it was undoubtedly a decknamen for an earlier British Arthr(h)i or Irish Artri.  This could have been a name or a title held by Ceredig. 

If this is what happened, then Ceredig could still have been an actual son of Cunedda.  And, historically speaking, Uther Pendragon/Illtud would have absolutely nothing to do with Arthur.

I've come to this conclusion because I'm as certain as I can be that Ceredig son of Cunedda/Cerdic of Wessex is Arthur.  I cannot reconcile this with Illtud as his father.  So I can only assume that Uther Pendragon as a title for Illtud was wrongly employed by the myth-makers as the father of Arthur.