MadameMinimes RP and Standard English are two different things. Standard English means grammatically “correct” English that is used for formal communication. You can speak Standard English with a Glaswegian accent and people with RP accents will regularly use non-standard English. There’s plenty of posh slang and/or jargon used by people with RP accent. Standard vs non-Standard is about what you say, not the accent you say it with.
Thank goodness someone has made the distinction! I have a posh Edinburgh, definitely Scottish accent, and say "Glass-gow" with a short "a". Thats still affected (although not as much as the ridiculous "Glarsgow" and I think "Glazgow" is more correct. It will depend on the letters following the "s" as to how its correctly pronounced but I think the "z" sound is more correct.
Pronouncing it with an "r" turns the first syllable into a long syllable rather than a short one, when both should be short syllables because they both begin with a "g" (two "g"s in one word generally bounce off each other as two short syllables") so the "r" sounds is all wrong.
Annoyingly, I can accurately mimic and therefore adopt an English accent, but strangely it comes out very, very posh. I don't like it! I think thats why so many posh people mumble or use vocal fry, so that it transmutes the class connotations of the accent. To me, adding a mid vowel "r" is trying to ape a posh accent but it comes across as a little fake.
The Queen says "Glasgow" in that clip.
RP changes all the time. Apparently English wasn't even so universally non-rhotic in England until televisions began appearing in most homes and people started copying newsreaders' pronunciation. Prior to that, as another poster pointed out, a posh accent was achieved by some by making "a"s sound like "e"s. Almost like proto-New Zealnders...
My favourite RP inaccurate pronunciation is the difference between "boarder" and "border". Apparently, amongst many RP speakers, they are indistinguishable, so intent are they on suppressing the "r" this time! Adding in "r"s where there are none and removing them where they should be! How bizarre! And how difficult that must make it to correctly pronounce many foreign languages, including all of the Scandinavian languages, German and French 