Tattoos

This has never been conclusively demonstrated and may just be justification after the fact. True or not, the upper classes see it as a lower class habit, possibly adopted by people who couldn't afford decent porcelain and so poured the milk in first to prevent the cheap cups from shattering. They find it hilarious that Middleton types think of it as a mark of good breeding, when (for them) MIF = OIK.

Orwell was Milk In Second purely on the basis that it gives you more precise control over the proportion of milk to tea.
This is, of course, correct.

Why would anyone in his right mind add milk to tea?

This is, of course, even more correct.