Its been few years since I worked in the mobile phone industry. EE was still T-Mobile and Orange back then, so things may have changed, but I did have a few customers who tried to move from a corporate plan to a personal plan and wanted to stay on Orange. Back then there were a few phone models that would work with a special WiFi router. A sort of rudimentary Wi-Fi calling, but it was only on Orange at that time. A PAC meant people had to move to another network, even if it was via a cheap PAYG SIM. I remember people doing this because no other network offered coverage in their house. So even though it was a faff, they did it. I have a feeling your company is giving you a PAC because once used it closes the account and generates a final bill.

I would have a look at Lebara (Vodafone)/Lyca (EE)/GiffGaff(o2). They are cheap as chips, have great roaming and international call options, if those matter to you. Some even offer eSims now, I see no advantage to the "Big 3", if you are buying a brand new phone Apple offer 24 month 0% finance via Barclays. o2/EE/3 will try to see a 36 or 48 month contract, which is a long time for the battery, if nothing else.