I’d imagine that the National Rail Museum archive might well have information on the numbering series. I believe that most publicity was also numbered, sometimes with a region code.
There’s probably also a list of numbers used for Network SouthEast & Intercity publicity; if I recall Network SouthEast used an NS code. I believe that Tony Hillman who runs srpublicity website had information on the NSE numbering however, it’s never been added to the website.
Post- COVID it’s probably worth contacting the Rail Museum archive to see what information exists as it’s an interesting topic with a potentially wide audience. It’s also one of the few aspects of railway that haven’t really been explored.