I do love Milly's posts.
This is an extraordinarily British thread, isn't it? Debating the finer points of what class means, and which class(es) we each belong to 
I worked in advertising/media. The standard job-based scale of A,B,C1,C2,D,E is still used but rarely on its own - it's about 70 years old. The industry uses blended methods, which are sophisticated and very good at reflecting consumer groups as we define them culturally. What I mean is, there's no statistical model called a "Yummy Mummy" but she can be very accurately identified used blended targeting. Until now, the biggest 'tell' was a person's preferred newspaper. Now people aren't reading paper any more, there's a bit of a scrum to construct a workable online equivalent.
All of which is to say, 'class' is a major red herring. You can discuss it till this thread blows up, but the fact is we all know what it means - well enough. Like some others here, I'm very class-mobile and suspect this comes from having a posh mum and a chav dad. I can convincingly "be" any British class except aristocrat and bottom class (as an aside, those two groups share many social & language quirks). I'm currently living in Nowhere on benefits: I look, behave & live the way my neighbours do - it's quick & cheap. The women & girls on my street do not live like anybody in "Mumsnet" land as sold to advertisers. They're stressed, pregnant and struggling. They have health problems. Their electricity gets cut off. There are multiple children per bedroom and only one bathroom, which is downstairs next to the kitchen. There is less than one car per household.
I think everybody here actually understands this; the thread's got hung up on what class means because ... ?