@teo_audio - you aren't off the wall on this. Discogs has pretty detailed info about EMI pressing plants and also covers the Philips/Phonodisc plants.
I know from the history of Island Records (which did own a pressing plant at one point that was previously owned by EMI) that Island did use Phonodisc/Polygram and switched to EMI at around the same time that the pink label changed to the pink rim. (You can tell by the deadwax but it's not definitive since in the UK some older metal work was still in use- thus, UK pink rims with Phonodisc/Polygram nomenclature). I know Island UK also used Orlake for some of the pink labels- yet again different nomenclature- an independent plant as far as I know. As to ex-UK, a whole other issue.
There was also a thing about getting early UK Harvests without the EMI logo- I have a few of those.
The relevance of this was that when the "youth explosion" really took off- no more Perry Como, but post-Monterey Festival-- Island, a small independent label was eating the major labels' lunch. Chris Blackwell had an ear and was tuned in. The big labels needed to create imprints for this more "progressive" (as used in the past, not all "prog rock" as we know it today). Thus, Deram (Decca), Harvest (EMI Group), Vertigo Swirl (Philips),among others.
For me, this is constant, ever continuing learning experience.