Like most of UK airlines, VS requires a CAA UK issued license.
There is no legal obligation to require this (even if they have spell it out, on their OM A chapter 5, the requirement for crew to have a CAA UK license) but they choose deliberately to request one.
On the contrary, and I am surprise that their legal dep does not advise them promptly on this matter, this is a blunt and ILLEGAL (for UK and EU laws) discrimination.
Problem is, nobody challenge it. So, airlines get away with this crap. And some other crap. But you know what your fellow collegues would tell you? if you do not like do not apply/leave.
We are pussy. And they know it.