English "Pants" is quite a recent thing
"Pants"="Trousers" is correct english, and remains that way outside the UK, including Australia, Canada, New Zealand, Fiji, etc. But colloquial english in most of the UK now regards pants as underpants. This intrigued me when I first arrived in London and over the next 20 years (I'm now back in Oz) I kept a sharp eye out for how it happened.
I've narrowed it down to starting in the 50s or possibly very late 40s (no occurrence in WWII) in the "upper" classes. But pants were still trousers for the "lower" classes until at least the late 70s. The source for the former, the earliest occurrence I ever found, is a story by P.G.Wodehouse in IIRC 1957, dealing strictly with the upper classes and actually playing heavily on the "dated" traditional usage of the word by Americans. The latter: a quote by Barry Sheene in IIRC 1978. I use "class" here strictly in the British sense. (From an Aussie perspective, Bazza was high class, Bertie et al were useless drips (albeit harmless and amusing).)
For pants=underpants to be declared fundamental (fnarr) --at least in the PLU subset-- in 1957, it had to have been a growing thing for a while. But that subset of the population featured prominently in British officers in WWII and there's not the remotest suggestion of such a substitution of meaning in anything written by them at the time or after. Quite the opposite -- pants were trousers (or shorts -- "short pants"). So it must have started some appreciable time after then. But it also needed to have been early enough for P.G.Wodehouse's story not to have been met with blank incomprehension when it was published -- which it was not. So I'm guessing around the cusp of 1950 among "the smart set", fawningly taken up in the usual UK-mass-rush by the wannabe smartset AKA the chattering classes. Throw in the British penchant for dressing up like and aping people they regard as high status (wanna buy an ordinary motorbike covered in racing stickers and random bolt-on "racing" accessories? go to england -- they're a dime a dozen), and hey presto, you have a cultural movement, leading to a cultural shift to a new equilibrium.
The fact that Barry Sheene was still automatically assuming pants=trousers 20 years later is a valuable data-point for all cultural anthropologists and linguists, in developing models for speed of cultural change.
Re the latter, the north-of-England's (not Scotland or N.Ireland) resistance to this modern wankery is entirely to be expected.