If you do, then you'd know that what I'm saying is totally correct.
And it should go without saying that operators obviously can and do work with Apple, but they don't have the luxury of getting devices to approve (and/or customise) and then give requirements back to the manufacturer before they range it.
Apple give a VERY short space of time, and if there are problems then the operator will miss out (imagine the damage of waiting until a future iOS/carrier update) and that's just not going to happen. What will happen is that the operator will delay launching a particular feature/service (e.g. WiFi Calling, VoLTE) and, obviously, there's no specific handset branding to worry about.
Perhaps by not working for a UK network, I have an advantage in that I get to speak to people from all of the UK networks (admittedly, O2 of late has been a company I've had almost no dealings with). O2 has an interesting procurement setup, or did, but I won't elaborate any further!