Where was Valentia Province in Britannia ?

[Robert Vermaat]

What about the area between hadrians wall and the Antonine wall?

Discussed in my 08:25 post above Byron.
IF Valentia had been located north of the wall, it would have been undefended according to the Notitia Dignitatum. And while the idea of a buffer state is by no means incomprehensible, it would be incomprehensible that a normal province that bordered the main enemy of the diocese would harbor no troops.