That's really informative. I'd dismiss the 1917 thing pretty much straight away, as things evolve, especially language, and imagine very few of us sound like we would have done over 100yrs ago.
Dismissing it straight away is pretty silly; it gives us a very strong view of where an accent came from and how it developed, and in particular what its geographical base is - particularly when you look at the consistency over the intervening years of class being the dominant non-geographical factor in the development of the accent.
Those aren't the only two sources; I mentioned both to show the remarkable consistency over at least a century of where RP is from, and what's it's 'for' in the eyes of many.
South of England is very broad too, and contains many regional accents. It's weird that it's stated as the standard, for the lower (third?) of the country, when there will be as many regional accents as in the middle third, for example, but the middle third doesn't have it's own "generalised standard" like RP? Or does it?
There is no generalised 'standard' for other geographies and you're right that this is because of the traditional cultural (class-based) dominance of RP/SSE. In some estimates over the years RP speakers in England are estimated between 3-10%.
South of England is very broad but I don't believe it's intended to be either geographically inclusive or restrictive; it's an attempt to describe an accent in a way that is less loaded than RP.
And there are variations as I said upthread - the eastern Home Counties have a more Estuary tone, the western HCs are quite different. And then there are, as you say, many other dialects within the broad southern English geography. Other people have suggested other names for it, such as 'General English' or 'General British'; none is particularly ideal.
IPA seems to use 'Standard Southern British' and notes
Standard Southern British (where 'Standard' should not be taken as implying a value judgment of 'correctness') is the modern equivalent of what has been called 'Received Pronunciation' ('RP'). It is an accent of the south east of England which operates as a prestige norm there and (to varying degrees) in other parts of the British Isles and beyond.
But I think you're now getting hung up on the name of the accent, rather than the fact that you have one, and that it is no more or less 'proper' than anyone else's.