Just to add a bit more to what's already been said here. ITV contractors all held stocks of programmes including standbys. When the entire ITV network was taken off by a 12 week strike in 1979, Channel Television was unaffected because the unions had a different agreement in the islands. Channel provided its own programme service during the strike, with lunchtime news, weather and what's on and then going on the air at 5pm with children's programmes, an hour-long local news magazine at 6 which included national headlines, and an evening made up from Mystery Movies, Columbo, New Avengers, Feature Films etc with a local news bulletin somewhere near 10pm. A huge effort on the part of the smallest ITV contractor, which worked extraordinarily well.
When the contractor went to black for more that thirty seconds, the transmitters did indeed take over with a caption. But there was a hot line telephone between the company's CTA (Central Technical Area) and the transmitter so they could co-ordinate a resumption of programme under whatever circumstances prevailed. I was on duty once when a power cut took STV off the air, and the transmitter switched the feed to Grampian until the power was restored.
I still have my copy of the old 'White book' agreement between the individual ITV companies and the ACTT, all the ITV companies are listed with the exception of Channel Television which as you say had a different agreement.
I have it in the back of my mind that during the 1979 ITCA/ACTT dispute Channel were potentially affected but the ACTT agreed that they did not have to walk out (i.e. they could continue to work) as to do otherwise would have bankrupted Channel which of course was in no-ones interest. That sort of implies that Channel were in dis-agreement with the ITCA within their own agreement. But it is all fuzzy in my mind now, and I'm prepared to accept I may be mis-remembering.