I suggest creating a specific topic in this forum to see what devices other members are using with Three UK. Confirm they have operational VoLTE.
Maybe also ask in the Three UK forums.
Then cross-reference with the lists of official and unofficial /e/OS builds on this site.
As for /e/OS vs. stock Android, it’s generally not relevant which is installed; if the original stock Android has achieved certification with your mobile provider to implement VoLTE calling, then it should also work on a custom ROM.
Caveats:
Samsungs, as you have found out, lose their VoLTE functionality if a custom ROM is installed. (That’s the current situation, anyway.)
I don’t know if there might be other phone brands that have a similar status to Samsung, so first research any prospective phone models you’re considering.
Speaking of phone models, note that the entire official model number is what counts when choosing a custom ROM, and when comparing it for compatibility and/or VoLTE-capability on a given network.
All good advice. I’ve been on the Three forums and I contacted support and they have assured me that my OnePlus 8 Pro will indeed work with the new non-3G network i.e it is VOLTE enabled.
If they have an online BYOD compatibility check, you can run your IMEI to see if it’s accepted, which will mean that VoLTE is operational/certified for the device on Three UK.
Probably obvious, but for any phone model you’re considering, besides network compatibility, you’ll also need to confirm that there’s an /e/OS build available for the specific model number.