Map of the Leadon Vale in Gloucestereshire and Herefordshire
I have looked again at the CULHWCH AND OLWEN account of the 'men of Llydaw', who are said to meet the pig-monster Llywdog at Ystrad Yw. The problem with this passage is that two personages from the Kingdom of Ercing are with them: Gwrfoddw, King of Ercing and his brother, Llygadrudd Emys - a corruption of the place-name Llygad Amr in Ercing (see P.C. Bartrum). This tells me that Ystrad Yw is an error for Ystrad Wy, i.e. Ystrad Gwy, the valley of the River Wye which ran through Ercing. The Vale of Leadon bordered on the portion of Ercing which lay to the east of the Wye. Ystrad Yw is over 30 kilometers as the crow flies from Weston Under Penyard, the site of Ariconium, which gave its name to Ercing. Thus the idea that the Ercing brothers would be in Ystrad Yw instead of in Ystrad Wy is nonsensical.
That Ystradyw can be corrupted into Ystradwy is proven by the case of the place-name Llygadyw in Powys (see https://en.wikipedia.org/wiki/Llygadwy). I thus assume the opposite could easily have happened.
I can now say with a fair degree of certainty that Uther Pendragon/Illtud was born in the Vale of Leadon and, doubtless, buried there. His connection with Brycheiniog is due solely to the presence there of a couple churches dedicated to the saint. If Defynnog does preserve a Dyfwn/*Dobunnos name, this may also have contributed to the relocation of Illtud from the Leadon to this part of Brycheiniog (as the Leadon Vale was within the ancient Dobunni territory).