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It feels like the UK is, while probably not the only one, definitely one of few countries where there isn't just the common regional and ethnic dimension when it comes to accents, but a 'poshness' one as well. RP or whatever version of that we'd now see as 'posh English' is something I have never heard any 'non-posh' person speak, no matter where in the UK they came from. You're definitely correct in that it's associated with the south of England, but to my knowledge it doesn't actually exist as a regional accent there either, right?
I assumed it was because the whole point of RP was to try to create a standardised English accent that could be understood by anyone across the whole country/empire/world