There’s a myth that the UC are faultlessly polite. They are, to your face, but in private they rip into the habits of the lower orders mercilessly. You are born into their world, or marry in to it but if so you never belong yourself, only your children do. Private school is tribal and thereby not optional, if you’re in their set, state is not considered ever. A social peer group know each other from pre prep. You may be invited to their social activities but you’re transitory so no one takes much notice of you. Unless you’re like an exotic pet, eg an outstanding sportsperson of colour, to flourish their ‘acceptance of diversity’ credentials.
my uncle by marriage was UC, my aunt not so, and I observed her being pilloried by her in-laws, and even her spouse, over decades. She was beautiful and vivacious and he handsome and charming but over time it was not a happy marriage. My aunt was my guardian while I was at boarding school so I had regular opportunity to ponder during exeats and holidays. You can’t just hold a knife and fork in a different way or change the name of the evening meal you eat, there are a million tells and you’ll not ‘pass’ if you aren’t of them from birth.
I am Lower middle class, you won’t separate me from my coasters, I twitch when a cold glass condenses water and leaves a ring on a decent wooden table 😁. I am clever, very well educated and financially comfortable, I can ride, ski and sail and I’m at peace with my place in the pecking order these days, but for a long time I felt like I was looking from the outside of a fish bowl to the inside. I can hold my own in socially elite company but I know I am irrelevant to them and their world is not mine. There really are bands of social groups and surprisingly little interaction between them in England in IME. Deciding you want to start eating avocados or holiday in a cottage in Cornwall has bugger all to do with it and will make no difference overall.