Lola I daresay you're being deliberately obtuse.
This thread has been an opportunity for people to pile in with their competing class-blind experiences, most of which are probably quite different from what the OP describes (university friends hardly count - the whole point of university is throwing off the shackles of your childhood, re-defining yourself, doing new things. Not to mention everyone is roughly the same age).
Surely it is extremely unusual for mothers bearing obvious, polar opposite class indicators to mix with ease.
Then at DCs school I felt the other end of it where many mums had au pairs, lived in massive detached houses etc and we live in a semi and (just) have a cleaner. I once went to a dinner at one of their houses - all talking about au pair experiences etc and I couldn’t keep up! They were all fine individually but as a group - not for me!
Oh yeah. I would say my random experiences tend to be more like the OP's. However, my son is at a fairly famous boarding school and we wound up at a party in Norfolk through some random happenstance. It was a concentrated distillation of upper-middles and uppers and I was a fish out of water. E.g. discussion of artwork, and also, everyone there was second or third cousins. Fucking weird.
I have a fair few upper-middle class friends, one of my closest friends is upper-middle, but I really do not want to go to an upper-middle/upper party again. It is bewildering.
I was so glad to leave and return to middle-class land.