I am reading U and non-U by Alan S.C. Ross, written in 1956. He wrote that the preposition on was non-U in the following sentence:
She's on holiday
This made me wonder what the correct U preposition or other expression was. I tried some prepositions in Ngrams, like at, with, and in, but none made me any wiser; I saw mainly noun adjectives and other noise. Does anyone have an idea what other preposition or expression might have been used?
on holiday, U, non-U)