As a former Southern man, I still think the BR all steel 4 Sub merits a model, and with minimal(ish) tooling changes (driving cars) the (SR pattern) 4 EPB could also come out of that, even if only in their 'basic' range perhaps? Suspect many modellers on a budget would be happy with a model that just represents the basics of what they remember, ie; cab front (driving car) appearance etc., not that I know anything about model making tooling you understand !
Taking this a step further, how about GE area EMU's, 306 etc., or (taking a nod from other sections of this forum) the Merseyrail old school units (28xxx/29xxx)? Was about to suggest the production era PEP stock, 507/508 etc., but is someone already making those, in N gauge perhaps? I'd buy a 00 gauge RTR unit of the SR/BR(S) era anytime, and do have an example of most that have been produced to date.
A tad ironic perhaps that those units that played such a major, if unrecognised, part in the lives of so many people (enthusiasts included) over the years are seen as not worthy of representation in the model form, although I do very much appreciate we are talking of an area/mode that in todays world has a very finite market potential.