There's a fairly new Playout/TX system on the Beeb channels.
It might be that it's difficult to re-programme junctions and abort programmes, when they aren't expecting to have to click 'Take Next' to move onto the next item in the schedule. Horizon would've had a fixed end marker to go into a Trail sequence.
Then the system is not fit for purpose for running a set of national reactive channels of a ‘state broadcaster’
Perhaps they should have let the pres OB truck at Wimbledon look after the junctions, with the affected programmes run off EVS on site !
Rich Tea and dvboy gave kudos