As an aside, after seeing your post here and learning some terms I didn't know before, this came up in my Google News feed today:
Accents are constantly changing.
theconversation.com
Yeah it's a good but slightly weird article because it's making a
strong distinction between Standard Southern British English and Received Pronunciation, and... there just is not a strong distinction that I am aware of. That's part of why people complained about Received Pronunciation - because it was really just Standard Southern British English. They're saying "Oh we didn't find any Received Pronunciation" and it's like, yes you absolutely did, you just called it something else and haven't explained why! I also notice they linked a definition of SSBE but not a definition of RP. Looking it up seems to suggest they're synonymous too. Hmmm.
This site about accent bias in the UK for example discusses it as follows:
Accents in Britain A nation defined by the way it speaks George Bernard Shaw famously wrote: “it is impossible for an Englishman to open his mouth without making some other Englishman hate or despi…
accentbiasbritain.org
“Received Pronunciation”, “Queen’s English”, “BBC English” or “Southern Standard British English” are all labels that refer to the accent of English in England that is associated with people from the upper- and upper-middle-classes.
I would slightly disagree because I would suggest actually traditional (aristocratic/generational as opposed to
noveau riche) upper-class people have a
distinct accent from RP, which is in fact, broader and slower in its use of vowels, and generally involves a bit of a drawl, and which is even more rarely heard on the TV or radio than regional accents (but definitely still exists IRL). Whereas upper middle class people reliably speak RP/SSBE in the South.
The article is a generally good one, but I think fails to grasp a couple of things - it talks about how MLE and EE are looked down on - and to some extent this is true - but this is the UK, and everyone eyebrow-raises at other people's UK accents (less so non-UK accents as that might be flirting with racism) to some degree (as a generalization), so the idea that it's a one-way street is a bit silly. I think the reality is many people in Britain actually don't have a single fixed accent they use 100% of the time, but rather at least a couple of accents that they use in different situations. This has been discussed particularly with Black people (Sorry To Bother You does some amazing stuff with the US equivalent of this phenomenon), but it's much broader than that. If I spoke like I did at home, at school, when I was at state school, I'd have got bullied! Even at private school, kids tended to have or affect different accents for at school and "in front of their parents".
Amusingly this is true for upper-class people too - the broad drawling accent I described isn't seen as appropriate for business, and engenders mockery, so they too conform to RP/SSBE when at work, for the most part.
(My man Saka in the video in the article is very definitely speaking MLE, so that's a helpful illustration of that. Both your article and the one I linked are definitely correct in terms of the sounds they're describing for the accents.)