Where was Valentia Province in Britannia ?

[Robert Vermaat]

The name of the new province, we may be certain, derived from the emperors Valens and Valentinian.

And definately not from 'Vallum' associated with Hadrian's Wall?

How would you render 'Vallum' to 'Valentia'?
Besides, this is what Ammianus says:
Res Gestae, Book XXVIII, chapter 3

… He [Theodosius] restored cities and garrison towns, as I have said, and protected the borders with guard-posts and defence works. The recovery of a province which had fallen into the hands of the enemy was so complete that, to use his own words, it now had a legitimate governor, and the emperor, treating the matter as a minor triumph, decreed that henceforth it should be called Valentia.

So yes, I would rule that out, definately. :wink: