Alexa
Received pronunciation was and still is to a large extent considered posh because it was the form of language acquired through birth or education by the rulers of society. These elite groups were to be found in the south of England, close to the seat of government, and among old aristocrats and newly rich who sent their children to schools where they were trained in received pronunciation.
Richer people, aristocratic landowners and industrialists, from the North , sent their children to schools where received pronunciation was taught.
Yes, historically that is correct, but there are as many 'common' people in the south as there are 'posh' types in the north, so it doesn't hold true nowadays, other than amongst those who, as you say, have lived in the south at school for all of their formative years. A friend of mine has a broad Eastenders accent, but insists that when she goes to the north of England people consider her 'posh', which I find very difficult to believe.
As I said upthread it is interesting how many people consider themselves accentless when this is far from true. It is as though they only hear accents when they deviate from their own.