[interestednovice]

I imagine the simple answer is that the setup as it is now allows for maximum redundancy.

Network (London) is the main feed that all others derive from, unless paired with Network (Salford).

Network (Salford) is a completely hot standby for Network (London) and vice-versa.

Nations then opt out of the sustaining network feed, so if they have a failure which means their opt doesn’t work, the worst that can happen is you see a sustaining feed (England’s programming) instead. Rather than a collapse of BBC One.

To cover anything other than an ad-hoc junction, the nations would probably need for equipment and personnel.