Hahaha. That's how they present themselves, but (at least in my class) that wasn't the demographic at all. Maybe two of that ilk in a group of ~18. (Pity, really: I have a strange affinity for that sort--and the Girls With Pearls too, if I'm honest. More information than you wanted or needed, hooray.)
Anyway: a good number of expats, European and American predominantly. Mixed group of mostly professional British women otherwise.
As for the NCT vs. Bump thing, a lot of people seemed to be there because, bluntly, they thought they might have more in common in that sort of socioeconomically self-selected group (being able to talk about things like private care, night nurses, nannies, long haul travel with newborns, etc. without anyone raising an eyebrow or giving them a hard time, for example) than in a randomly assigned one. (Though given that most people seemed to live in RBKC that made no real sense.)
Ultimately, I hated the idea of all those groups--so not my thing. Way too cool. 
So as I said, I participated grudgingly. Very.
And that was very, very stupid.
After having a child and realising that the only people who were going to really get me were women going though the same ridiculousness at the same time...well, I wished then that I'd been more engaged.