Hi Perryman - these are interesting points you make
"The road that historically connected Peckham to Sydenham perhaps back to the Roman times is Wood Vale which is mostly wide, flat and straight.
But the residents on this road are even richer than those on Manor Mount and have long ago successfully repelled through traffic and bus routes."
Doesn't Wood Vale still offer this route? I think it does, but not the other way around - Sydenham to Peckham - as there is a No Right Turn into Wood Vale from the South Circular. Also Wood Vale has a No Right Turn into the South Circular towards Lordship Lane / Dulwich - there would need to be traffic lights to facilitate that turn across multiple lanes of traffic. This may also be why the road has not been used for a bus route, although the 63 does a 180 degree turn on Forest Hill Road junction with the Northern end of Wood Vale.
Not wanting to be cynical, but perhaps you have a point that the wealthier streets tend claim designation as 'residential' and 'unsuitable for through traffic'. I think this is happening across London, but it doesn't mean everyone has to put up with it.
In several locations in and around Forest Hill, it is hard to see the situation continuing as it is now. Dangerous as it may be to drive (illegally) into Manor Mount from Honor Oak Road, it appears far more dangerous to attempt a (legal) right turn out of Forest Hill Station car park - driving into traffic which is unsighted coming fast around two bends, also lots of pedestrian activity. The junction at Waldram Crescent and Devonshire Road is also extremely dangerous.