Class has nothing really to do with "posh-ness". Posh is a relatively new expression coined by those who could not themselves afford to travel first-class Port-Out-Starboard-Home across the Atlantic on the fashionable ocean liners, so they disparaged those who could.
It has been extended to cover any nicety the speaker does not practice themselves. In "Hi-di-hi" Gladys asks, on hearing that someone is getting married, "When is the baby due?" No baby? "There's posh!!!"
Fish knives were originally a different shape to distinguish them from the usual ones so that only they were stained by fish - no stainless steel so they had to be arduously cleaned each time used) If you could not afford extra knives you used the same ones for everything.
Linen napkins kept your expensive clothes clean. There was no washing machine to throw them into. If you were a scruffy peasant or a slob you didn't bother.
I would define anyone's real class as how they treat other people's feelings, wishes and - yes - foibles. The "upper class" can be as good as that as the rest of us (or as bad)