The News Channel is what is carried on BBC1 SD when the regional news is on. If a region doesn't opt or has to crash out of the opt then that's what is seen.
If its known in advance then an alternative region can be routed out instead, but as I say that takes a bit of notice
They should bring back what they did in the 70s before there was any London News and have a Tom and Jerry cartoon as the sustaining service.
But seriously, wouldn't it be better in this age of server playout just to have a lump of trailers as the sustaining feed, (at least for the short opt slots)?