Chris,
Congratulations on trying to explain the full Southern coach set history, it is something I have pondered over many times when renumbering stock to make different sets out of existing Hornby models.
I've made one discovery since re-reading your notes on the 2-Coach 'P' Sets and that is that I have put the wrong number on the BCK in Set 25 on more than one occasion; I really must get my glasses fixed!
Thanks for pointing out the source for the Bill Bedford sides, I wondered where I could obtain these.
Of course the easiest way to make different sets is to renumber the Hornby models, but they have not produced all the diagrams across all the livery combinations (yet), most notable is the lack of High Window stock in lined SR olive (other than in train packs). This is the most complicated livery to reproduce, so repainting and lining is not an option unless you are a lining expert.
HMRS produce the coach numbers and Set numbers for SR liveries, I use the yellow Fox transfers or HMRS for BR green coach numbers and Fox 8" off-white are the best size match for Set numbers in BR days.
Glenn