Double barrelled surnames used to be posh when I was young. No longer the case. Quite the opposite.
It also used to be posh to have a personalised car number plate, now every Tom, Dick and Harry ( who were they btw?) can have one, no longer posh.
Hard to define ‘poshness’. My DD used to be a nanny to a lady who epitomised being ‘posh’. Not just how she spoke, the lovely house she lived in, her lifestyle.
If she had lost all her money, her home, her lifestyle, she would still have been ‘posh’, but I can’t tell you how I know that.
My mother would have called it ‘breeding’.