RP is not generic southern English I can assure you. There is no such thing as generic southern English any more than there is generic northern English. There are broad speech similarities that mark someone out as from the north west, or the north east, the south west, the south east, or the midlands. That's all.
By 'southern' England I presume you are dividing England up into three broad bands of south, midlands and north. So everything south of the midlands would encompass all the home counties as well as Beds, Oxfordshire Hampshire, E and W Sussex and Suffolk as well as all of the west country south of the midlands including Cornwall, Bristol and Gloucestershire? That's a huge sweep. The idea that they all speak in a way that is some sort of generic accent that is loosely RP is just so inaccurate I don't even know where to start.
People in Newcastle don't sound anything like people in Manchester but by your logic they are both 'northern' so they must speak the same. 
There are subtle differences between the fairly bog standard accents of Essex, Kent and Surrey that I can discern because I am very familiar with all those areas and people from them, but those differences may not be picked up by someone from Newcastle for example. To them they'd just hear 'southern' or 'London.' If they think they hear RP then they don't know what RP is.
There will be people who claim they speak with RP, by which they usually mean they ennunciate clearly, probably have softer, more modulated vowel sounds compared to others who speak with a much stronger regional accent than them, don't drop their Hs or their Ts etc, use standard grammatical sentence construction etc. But that is not the same thing as speaking RP. They could still very much be identified as 'northern' or from or the west midlands or even specific parts of Scotland, even if they are perceived as quite well spoken or 'posh'. RP is not the same as simply speaking well but with a regional accent, however refined it is. If it's identifiable as regional at all then it's not RP.
RP is a very specific and prescribed way of speaking English. Anyone who speaks it in its pure form could be from anywhere in the UK and you would not know where. There would be no giveaways to a specific region in their speech, or only barely perceptible ones that a trained linguist would be able to pick up on, but not your average person. That's the whole point of it. It's a way of standard pronunciation that makes English clearly understood by anybody in the world, without the need to further interpret or understand an added layer of regional variance or dialect. I've heard Aberdonians and Scousers interviewed on TV where their regional accent was so strong I felt I needed subtitles even though I am a native English speaker and so were they. If they could understand me perfectly in a reverse situation, it's not because I am from the south, it's because I speak quite 'well' so while I don't speak RP, my local (greater London) accent is a fairly soft one. But put someone with a very broad working class London/Cockney accent in front a a foreigner who thinks they understand English well and it could be a very different story.
RP is supposed to remove all that confusion and be the default for spoken English. The 'better' you speak, the closer it will be to RP but that doesn't mean it is RP. But 'better' in this case simply means more easily understood by the greatest number of people.
Traditionally most people who have grown up learning to speak with RP have not had much interaction with the locals of their area, being from the sorts of families where everyone was packed off to boarding school then university, in the days when uptake was more like 7% and less like 50%. By the time they were fully formed adults their accent would be fixed and not 'adulterated' by the influence of regional speech. And the working class and LMC kids who made it to university had probably had their regional accents knocked out of them (or at least softened) by grammar schools and scholarships to private school.
These days RP is seen as unimportant, as class barriers break down and regional accents are embraced and celebrated. RP is certainly no longer the key to the door of certain careers. Although undoubtedly, being able to speak 'well' will still help in certain spheres. But 'well' does not mean without any trace of regional accent.
Except TV and radio, where these days the very opposite is true and RP is more likely to hold you back and mark you out as someone who has succeeded not necessarily through meritocracy, but privilege, nepotism or old school networking. Whether that is true or not is beside the point, it's the perception that matters.