Isn't that down to the idea of 'nominated contractor' - having one region leading the network. If the usual one was off air surely that status would just move to another? AIUI the role did move outside London every so often to give the other big regions a bit of practise doing it
Both Central and Granada were quite active in feeding the network too (CITV and Schools passing through Central five times a week usually, and the morning schedule post-1988 always going through Quay Street).
As an aside, I wonder why ATV and later Central became responsible for networking the schools broadcasts in the late 60s - I think it was the responsibility of Associated Rediffusion prior to that.
Also - and more on topic - when a programme couldn’t be transmitted due to the originating company being on strike, why did the programme have to be replaced independently by each station? When Thames were on strike, Granada couldn’t decide to network something instead. I assume Channel was excepted from this rule, taking TSW’s local replacement programming in such cases? I’m trying to remember what viewers outside London actually saw instead of the many Thames programmes that were pre-empted during the 1984 disputes. Probably American dramas/comedies or repeats of UK shows? Companies like Central, Granada and YTV would have plenty of this kind of material, but what about the smaller players? Were they allowed to buy something in from one of the “big three” to show - assuming they weren’t allowed to make a local arrangement to latch onto their broadcasts?
Cut out the coupon in your TV Times!