@RosesAndHellebores
I think it's quite interesting. Take DS for example: he has curated being a bit "London lad", is left leaning, wears black jeans, trainers, t shirts, hoodie, 26 now but did 2 gap years with a "trendyish" start-up company with a young sales and marketing team. He was known as "posh boy" - good humouredly.
His interests are music, art, books, politics, sport, including playing football. His colleagues had no way of knowing his background.
But they don't need to. Just as he doesn't need to be in the studbook to code as 'posh', at least to certain people, like these colleagues.
I mean, it doesn't mean they're correct in any objective way (assuming you aren't in fact landed gentry) -- it just means that in a society where social class is absolutely entrenched, so that people make class judgements about others without even being aware of it, or that they are class judgements, something about him came across as 'higher up the social scale than I am' to these colleagues.
It depends on the perceiver's social class, and their life experience, and their ability to recognise social codes and shibboleths.
To some people I code as 'posh' simply because I write for a living, have a lot of friends in the arts, and love and am knowledgeable about literature, painting, opera etc. It's not true in any objective sense I grew up with an outdoor toilet and an illiterate parents, and was the first person in my family to finish secondary school, far less go to university, discovered these things on my own, and DH and I don't have pots of cash even now but it's because I'm perceived to have a lot of cultural capital.