I'm sure TonyCurrie's spoken about it before. What would happen is the union would 'black' a station, which literally meant the station's output would go black, and after 30 seconds (I think?) the IBA would automatically retake control.
Yes, and after the local IBA control centres ceased to be manned (circa 1990) if the primary transmitter lost the video sync pulses, it would either automatically shut down, or radiate an apology caption, depending what time of day the signal failed ! The IBA were always watching and monitoring remotely all ITV and C4 regions, from 1980 from ROCs at Black Hill, Emley Moor, Croydon, and St Hilary (S Wales), and from about 1993 just from Emley.
In fact with the privatisations, and subsequent mergers of the IBA and BBC transmitter departments, just about all terrestrial TV transmissions are now monitored in some way from Emley. The only exceptions AIUI are the remaining analogue BBC TV transmissions, that are still monitored from the legacy BBC site at Warwick.