[cable]

Yes, I remember getting those stickers. I think I did use them too...

It isn't so much the physical network but the logistics that would make it a major undertaking (esp. if there are complications like customers with static IP addresses) which is probably why they haven't done it yet even though there are benefits, as the BT broadband network clearly has been the focus of investment and with bells and whistles such as IPv6 which EE and Plusnet never had.

Given the emergency alert debacle, I don't think anyone's going to take network engineering lessons from Three any time soon

end user DHCP and Static IPs would be relatively straight forward if the routing was in place. It would require changes at layer 2 and probably require uplifting whatever is acting as the router/gateway (possibly an SVI if CISCO hardware) and ensuring VLANs are added into the the new hardwares VLAN databases and trunked back to the core network.

Threes 5G network is actually very good. That emergency alert was a test after all, it turned out to be some software bug that prevented reception.

You are right though, it is a big undertaking but perhaps more common than you'd think. Integration of IT systems happens all the time.