I investigated N for a forthcoming project myself, having found out that 3D bodies are now available for a number of Irish diesels and Craven and Park Royal carriages. At that scale, while not technically correct, one or two varieties of British goods vans would have been (for me, anyway) a reasonable approximation to "H" vans and open wagons. As you say, Tony, much greater possibilities exist in this scale. The only downside was the amount or preparation and decoration of the somewhat crude 3D prints, as i was advised.
I have little doubt that with more and more people living in ever-smaller accommodation, N will resurge in the years to come, and maybe then some Provincial Wagons / Murphy Models / IRM / SSM etc etc lookalikes (if not those entities themselves) will be able to see a potential market. With your big GNR / UTA interest, there are RTR mode;s of various 0.6.0s which might be altered to look vaguely GNR - stretching artistic licence a bit, had the "Derry Road" remained longer, it's probable that NCC "Jeeps" would have become a common sight. there are various 2.6.4T locos which, again, might be altered to look good.
The UTA inherited a lot of NCC coaching stock, much of which was ordinary LMS designs. Thus, some LMS stock repainted dark green would be fine, even though it was mostly ex-GN stock on the Derry Road. As the saying goes, "there's a million ways to skin a cat"!