I agree they should offer it, but from reading, it’s a complex design and probably due to O2 infrastructure and systems they’re probably dated which is causing the delay.That's not looking likely tho that O2 will roll out IMS to PAYG users:
Post 180 from Giff Gaff say they will here: https://community.giffgaff.com/d/33325393-future-flexibility-we-want-your-thoughts/184
Post 3 from Giff Gaff here say they have cancelled the rollout of IMS:
So from that it seems that yes they were going to introduce it but something happened and now they have no intention of doing so ever which is a pity as £20 for 100GB data on Giff Gaff is more appealing then £30 for same data on O2 PAYG or Tesco Mobile PAYG especially as WiFi calling and 4g calling isn't available yet.
Note all operators as standard should offer IMS services regardless of who they are
I’m confident O2 will bring it to payg and whoever else, they just don’t have the full package, and rolling something out half complete will probably just cause more complaints than it’s worth.