To begin, let's discuss the beginning of the Vulgate version (https://books.google.com/books/about/Lancelot_Grail_The_death_of_Arthur.html?id=d-jFvu5lp8EC:
I've been asked by a friend to pass this along again:
https://www.library.wales/discover/digital-gallery/printed-material/a-welsh-classical-dictionary This is an invaluable resource for anyone delving into the 'Welsh Arthur." I have used it extensively over the years for research purposes.
For quite awhile now I have been bothered by earlier research on the true identity of Uther Pendragon. While I devoted several blog posts to the subject, one of the main ones can be found here:
https://mistshadows.blogspot.com/2017/12/uther-pendragon-discovered-at-last.html
To state the problem as succinctly as possible: Mabon son of Modron in the PA GUR poem as one of the vytheint (? - this word remains unknown in terms of its etymology; guesses have been made, but none are satisfactory) of Elei is unexplainable.[1] Mabon is not present in the valley of the River Ely (Elai, Elei). Because he is called the servant of Uther Pendragon, it seems unavoidable to instead seek Uther Pendragon in the Ey Valley, and to thus assign Mabon as his servant there. The Ely Valley is within what was the early kingdom of Penychen between the rivers Taff and Thaw. And that kingdom is where we find Illtud serving as chief of the soldiers of Pawl Penychen. Illtud is given military titles in Latin that conform perfectly to the Pendragon title of Uther, and Uther itself is a perfect Welsh translation of the terribilis (miles) applied to Illtud after his death.
Illtud is described otherwise as a cousin of Arthur. NOT his father. But we must allow for his being made into an important saint as having influenced his earlier life as a soldier. He is also made to put away his wife. I'd conclusively shown that the 'Llydaw' he came from was the Vale of Leadon, anciently part of the Kingdom of the Dobunni.
I dispensed with him as a father of Arthur for the simple reason that the hagiographical tradition did not allow us to identify him with Uther Pendragon. But since when should we trust the historical accuracy of a saint's vita? As his military past was downplayed, we might assume that his paternal trace to Arthur might well have been expunged.
Of course, it is also possible Uther Pendragon or Illtud was merely chosen by Geoffrey of Monmouth or his source for no other reason than that Arthur's real father was not known and Uther appeared in conjunction with Arthur in the MARWNAT VTHYR PEN. Or even that Geoffrey picked Uther totally at random. Any of this is possible, as Geoffrey is the first to make Uther Arthur's father and pretty much everything in Geoffrey's so-called history is patently fiction.
It is also possible that the Welsh relocated Uther Pendragon to Wales, and that they identified him with Illtud. Dr. Simon Rodway has assured me that the name + epithet formation of Arthur's father's name should not be seen as anything other than that. But the possibility that such a name + epithet could have been associated with Illtud cannot be discounted.
Regardless what really happened, we appear to be stuck with Uther in the Ely Valley. Mabon can't be placed there, nor can can his fellow vytheint of the PA GUR. The conclusion can only be that Uther belonged there, and these warriors collected around him in legend as his retinue.
What we make of the Uther Pendragon = Illtud connection is something I am still pondering. I think given the preponderance of evidence for Arthur in the North, specifically on Hadrian's Wall, and given that Mabon was a Northern deity, it is likely the name Uther Pendragon has fancifully been brought into connection with Illtud's Latin titles. And that meant that Uther was relocated to southern Wales, at least in the PA GUR.
We should bear in mind the the maendy or 'stone house' of Mabon in the MABINOGION, while placed at Caer Gloyw/Gloucester, is also a place-name found at Mynydd Maendy, on one side of the Ely River watershed, while there is also a Maendy at Peterson Super Ely. This part of Wales was once called Glywysing, for an eponymous found Glywys. This name means 'man of Glevum', i.e. of Gloucester. So there may be nothing to Mabon at Elei other than a standard folklore relocation.
[1] I tried very hard to find an Elai/Elei in the North, but have failed to do so. It seems unavoidable that this is, indeed, the Ely River in South Wales.