Oh, believe me, I'm not speculating, I have visited and I have asked lots of questions! I'm not guessing any more than anyone else is! But my investigations have raised the idea that PSC has expanded by 500 pupils, recently, which has had the effect of making getting study space harder, all of which makes me nervous! (I think the 'wet December morning/ where will everyone go?-' thing is relevant!)
And once again, many who tell me their DD is loving it tell of, if not exceptional academic ability, certainly of five AS levels and diligent performances- again, something akin to my perceived PS 'model' as detailed: female, clever, compliant, self-motivated. All of which DS, at 15, isn't, really! 
I do 'get' that PSC obviously isn't full of high fliers, but, frankly, if DS is destined for the "2 A level" trajectory, it would make far more sense to go local to BP as the PSC 'advantage' only kicks in for uni entrance level, iyswim. It's 'prestige value' only 'matters' when impressing university admissions people.
Anyway, we will visit both again over the next fortnight. DS has a personal mentor in place now to try and keep him on the straight and narrow and I suspect both PS and BP will be applied for!