Where was Valentia Province in Britannia ?

[Arturus Uriconium]

Although it's mostly been covered here, I thought this quote from the bookrags.com website worth an inclusion:

Another point is Ammianus' comment that the province was named by Valentinian as if celebrating a minor triumph (velut ovans). These minor triumphs were a Republican honour granted for unspectacular victories or defeats of slave revolts. A full Imperial triumph celebration would have been more appropriate for a defeat over the invading hordes and it is odd that the emperor was so restrained. One explanation is that full triumphs were never granted in victories over Roman citizens, the enemy had to be barbarian for a general to receive full honours. Possibly Valentia was a breakaway province.

It certainly looks from the evidence that there were already five provinces, one of which had been lost, and that that could be the Rheged/Coeling region. A region very important to get back because of the Wall. If it was what is now Wales then you'd want it back for the minerals, as well as pride. Same for Dumnonia. If it was north and west Wales, that now could in the hands of Irish or Goidelic speakers, then it could add to the Cunedag question.

But why list Velentia after Maxima Caesariensis in the Notitio if it's some out of the way province? Why is it not listed before or after Britannia Secunda? Have we got it totally wrong and it was actually somewhere northwest of London?