So, I guess the reverse would also be true, if the incoming feed was lost, then presumably the outgoing would be lost as well?
In which case what would happen?
If a station couldn't air a programme to the network due to a down 'out' line, would each company have to fill locally, or would someone like Thames provide a network filler?
Or, if there was plenty of time - get that VT
in a fast car to a neighbour region (Thames to Southern / Granada to Yorkshire etc) and get the programme to air that way?
In the days of analogue pal, the circuits in the ITV stations I worked at were supplied by a lead sheathed cables containing 4 'tubes' from the local BT switching point. Unlikely for all four to go down I would have thought, not impossible with a big JCB I suppose. If an ITV station was going to be off-air for any length of time I would have thought the ITA/IBA would arrange for a neighbouring contractor to be put to air as a replacement service - certainly that happened. Circuits were usually repaired very quickly by postie. I think, 'think' mind you, that the bigger Itv stations with multiple cables in and out that they may have taken different physical routing for rather obvious reasons once outside their premises.
If a gap in the network schedule was due to a technical fault (rather than a dispute) then it would have been ok for the rest of the network to fill the gap with networked material from another company. ITV was a very common sense operation (and a very complicated one).