RP started among the higher classes in 17th-centure England, then slowly filtered down to the rest of the populous. Essentially, most British accents, from Cockney to Manchester to Birmingham, are colloqualized versions of RP with tons of regionalisms thrown in.
It wasn't really the native dialect of the home counties, since the native dialect of London in Shakespeare's day was actually more of a brogue similar to the West Country or Ireland. Rather, it was a somewhat artificial, unnaturally refined type of speech that was spoken by the upper-crust, who were at the time actually more comfortable with French or Latin than their native language.
I would guess that a lot of RP's phonology was something of a hybrid of natural English and the almost foreign dialect that many royals would have spoken with at the time. For instance, the vowel [Q] in the word "not" might be sort of a compromise between the standard, flat [A] of Elizabethan English, and the unnatural [o] that a foreign speaker would read the word as. And the alveolar approximate is also something of a compromise, between the retroflex r and an alveolar trill. That would certainly explain the whole "non-rhoticity" thing, since [r\] virtually disappears when placed between a vowel and another consonant (without adding a schwa first). Just a guess.
It wasn't really the native dialect of the home counties, since the native dialect of London in Shakespeare's day was actually more of a brogue similar to the West Country or Ireland. Rather, it was a somewhat artificial, unnaturally refined type of speech that was spoken by the upper-crust, who were at the time actually more comfortable with French or Latin than their native language.
I would guess that a lot of RP's phonology was something of a hybrid of natural English and the almost foreign dialect that many royals would have spoken with at the time. For instance, the vowel [Q] in the word "not" might be sort of a compromise between the standard, flat [A] of Elizabethan English, and the unnatural [o] that a foreign speaker would read the word as. And the alveolar approximate is also something of a compromise, between the retroflex r and an alveolar trill. That would certainly explain the whole "non-rhoticity" thing, since [r\] virtually disappears when placed between a vowel and another consonant (without adding a schwa first). Just a guess.