I agree with all the practical on-the-ground implications as to why .303 + pistol-calibre SMGs were unmovable in OTL, but disagree that it was because the UK and Commonwealth had an oppressive economic or logistical impediment stopping all change in this area (certainly not after America WAS IN), but reupping this:
Ultimately, I think this is a military reform that only happens in the event of that hypothetical long war occurring after a Soviet collapse in '42/'43, plus the success of the Manhattan Project being knocked back a year or three.
At that point, increasing infantry effectiveness becomes a much bigger thing for Whitehall than it ever was IOTL.
Something to consider; the brasshats were fundamentally correct during the interwar period in selecting the Bren and having the Vickers Berthier as the leading alternative to it. They got that right, and any modern talk about "but they should have made 'em beltfed" ignores that the system worked within the context that existed.
A war with a vastly different strategic requirement and lengthened timeframe to OTL, that very easily leads to the implementation of our smallarms reform.
Anyway, my feeling is they wouldn't even be arming all infantry with autocarbines, they'd simply do away with 'all the king's rifle group men are equal' doctrine, and end up with like a third of the men in a section carrying bolt action carbines and acting as dedicated ammo barers for the autocarbines (autocarbineers?) and Bren team(s).
FWIW, plenty of No 5s were made IOTL, plans existed for No 4s being converted to halfstock carbine configuration, and the same can easily apply to No 6s and SMLEs in the event of a long war. This is before you factor in doing a downloaded .303 for these weapons alone, or simply chambering them in the same calibre as the autocarbine class.
but I've become wiser with more research
Off topic, but he really needs to turn his smarts into a regular PhD, his analysis is next level compared to the other brighteyed YT military history presenters.
Though I guess that career path in the gaming entertainment industrial complex is way more comfortable than throwing in to become a doctoral candidate in this day and age.