In all reality, the likelihood of a Park Royal behind an 800 class, while technically possible, is highly unlikely. By the time the main line PRs were being introduced, the 800 class were doing very little - Cork trains were largely in the hands of Crossley "A"s, B101s and AEC railcars.
The stock needed for AUTHENTIC stock behind an 800 is a selection of both types of Bredins, the 1951-3 CIE equivalents, and various older main line corridor wooden-bodied GSWR stock. The older wooden ones and the steel-sided ones (Bredins onward) were, judging by photos of anything steam on the main line in the 1940s and 50s, about 50 / 50 each.
With the actual absence of anything at all RTR at the moment, and (I suspect) for a long time, we must make do with what the nearest equivalents are. Apart from the SSM and Worsley brass kits, for RTR various types of 1930s off-the-shelf LMS designs from across the pond would do a reasonable impression of Bredins if painted in GSR livery (same as LMS, conveniently, down to the lining, but with very dark grey roof instead of silver) though different lettering; and there are some representations of wooden-bodied stock from Britain (go for LMS or GWR types, not Southern or LNER) again repainted. Alternately, the very same vehicles in older CIE green with eau-de-nil lines, as per the re-liveried LMS stock that came in that Bachmann train set with a Woolwich some years ago.
The 6-wheelers wouldn't have gone behind an 800, with the possible exception of a full brake carrying mail bags, and alongside a TPO; now THIS is something for which nothing but a scratchbuild would do, but would be present in most 800-hauled trains.
So right now, nothing beyond that; but who know what the future would bring. A RTR Bredin or laminate would cover a huge lot of holes in the market.