I suspect it was a posh way of distinguishing oneself from the local oiks by being "correct". I was born in the 50's 15 miles from Coventry. During the sixties, the pronunciation was occasionally discussed and used in both forms. The "Cuventry" form died out (for obvious reasons) by the early
1970's. A significant percentage of my grandfather's generation used that pronunciation but rarely the succeeding ones, so we are talking about what was common (in RP) pre-WW2 persisting afterwards. The same generation (at least those who frequented the establishments) would refer to an 'otel (again, probably to distinguish themselves from those who didn't).My father, who worked in radio during the war and on cinema sound systems afterwards, always pronounced "stereo" as "steereo", still listed as an alternative pronunciation, although he was from Lancaster and never said barth or grarse but bath and grass with short "a"s, so it wasn't a southern affectation nor ignorance of the subject, just what he was told was "correct" earlier in his life.
"Correctness" is a slippery beast, that may not have the parentage one expects. I prefer a dynamic language where different generations use different forms - it's more interesting, less snobbish and less judgmental.