All my furniture is second-hand, so I suppose that makes me very posh indeed!
My late aunt had been head parlour-maid in a titled house in London between the wars. She was regarded as posh, and as the arbiter of how things should be done. She always had pastry forks and matching china instead of odd cups and saucers like the rest of the family!
I can remember that she would tick us off if we behaved 'common', and would say 'It's not PROPER!' if we neglected our table manners.
Poor aunt,and her sisters, including my late mother, had been brought up in the most grinding poverty during the 1920's. They all went into service, and the grand houses that they worked in must have seemed like another world.
They were all fortunate in that they went to good houses where servants were treated very well, so they returned home to see their parents much better nourished and more warmly clad than when they had left. Many other girls had bad experiences and were worked half to death and poorly fed.
I don't know what POSH really means - I suppose it's all about perception. If you have nothing, then anyone who is comfortably off would appear posh, but I suppose that having to think about it at all means that we must be 'rather common' as my dear aunt would have said! 