That’s a really interesting question. I grew up in a working class area in the north in the 1980s, and had one ‘posh’ friend - her mum and dad were both teachers, they lived in a semi-detached house, they had two cars, and she didn’t have to share a bedroom. My other friends all lived in terraced houses (like me) or council houses, with dads doing manual work and mums who were cleaners, worked in a factory etc.
I went to university (back in the days of student grants and no tuition fees), so I have a professional job, as does DH, though we’re not in high-flying careers.
DH and I live in a smaller terraced house than the one I grew up in, though we do live in a more affluent city so houses are more expensive, so child me I wouldn’t think our house was posh. However, DD (age 10) is an only child, so has her own bedroom, more toys than I did, and her clothes aren’t mostly second-hand or handed down. We have the equivalent of Sky TV (streaming services). She’s had birthday parties at soft place type places, which were unknown when I was growing up. So that would make me think she was ‘posh’. We also serve ‘exotic’ food like pasta and rice - posh to a girl who was brought up on meat and two veg type meals.
We also call meals breakfast, lunch, and dinner compared to my upbringing of breakfast, dinner, and tea (DH’s influence - he is southern 😀).
My mum took me to the library and occasionally the local museum, but we also take DD to the art gallery, the cinema, and occasionally the theatre. DD has done more extra-curricular activities than I did, and has been out to eat in cafes and restaurants much more. She’s also had two holidays abroad, while I didn’t go abroad until I was 18, though we had holidays in the UK.
I think we would be friends, because we are very similar, both love reading, and are into non-mainstream things like science fiction. However child me would feel a bit awkward as I would perceive her parents to be posh (even though we’re not really!).