1.Well, the actual % of kids in private ed went up enormously under the Thatcher Terror of 1979-91 - almost doubled from about 5% to about 9%. The northern working class may not have done terribly well under Maggie, but the southern middle class most assuredly did. One can think of a lot of formerly petit bourgeious families who in previous generations might have eschewed the option on financial grounds, or because they thought that the snobby boarders were not for them: the Middleton family would be a paradigmatic example.
2. The whole of the UK pop/theatre/cinema/dance/arty farty/ TV/luvvie sector is massivley skewed towards the London area: where most of the rich kids already be livin'. Number of kids in private Ed in Surrey is probably about 25% - South Shields, it's probably 2.5%. Being proletarian and/or northern and/or /Celtic like the Beatles or Sean Connery or Richard Burton or Cilla, or the Kinks, or the Stones or the Who, just doesn't seem to be fashionable in Luvvieland like it was in the swinging sixties: all English actresses want to do these days is dress up in bodices and do Jane Austen.
3. Quite a few of these kids seem to come from already established showbiz families who came from more humble beginnings, but made money out of rock n roll/acting etc in the 60s, 70s 80s -Lilly Allen would be an excellent example: does seem to be lot of nepotism and networking going on there.
4. The public schools are there for a reason: to give their kids advantages. Zeitgeist is their middle name. Up till circa 1960, they were there to provide army and navy officers and colonial administrators. In the 80s (when I was in Uni - in some tutorial groups, the only non-privately educated kid there) everyone wanted to be Gordon Gecko (or at any rate the better spoken English character played by Terence Stamp ) which was regarded with horror by many of their parents, who didn't grasp that the empire wasn't there any more. These days, they seem to be deliberately targetting the creative/music/theatre sector as a place to send these upper middle class brats, which woulda been unthinkable in my day.
5. The Cockneys are all in Spain, having abandoned London to get away from the Effnic majorities - as they are now in the inner city - or they're hiding out from The Yard.