We have tested some NI registered vehicles for holiday makers in Inverness (I have a recollection of a strike at testing station in NI a few years back, people travelling to Stranraer in Scotland to have the test done), Cherised number plates from NI can also be used in other parts of UK so their registration scheme must be through Swansea, so I would assume a test done in Scotland or England is the same VOSA scheme as NI.
Sorry for going of on a tangent about plates HTH Graham