U-English of the 1950s: what was used instead of "ON holiday"?

Ross mentions it briefly in his 1954 paper, where he gives it as an example of his claim that 'Some phrases with prepositions are non-U', but he doesn't offer the U alternative. As David has said in his comment, it’s quite possible that U speakers would say She’s holidaying. Holiday is first recorded as a verb in the OED in 1869.

Two other examples of the non-U use of prepositions he gives are 'He's at boarding-school' and 'We were at table'. There, too, the U alternatives are not given, but my own suggestions are 'He's boarding' and 'We were dining', both of which, like 'holidaying', use a verb in place of a prepositional phrase. (Of the first, Ross rather grandly states 'In any case, boarding-school is little used by U-speakers, for, to most of them, there is no other kind of school.')