Getting away from supermarkets, I was wondering about the power failure in 2000 at TVC. With control switched to Birmingham, they were playing out Dads Army, which BBC South east were taking in place of their local news. So, how could Birmingham then have opted out of themselves to show their regional news bulletin? Were they able to do so?
They would just have patched feeds from the makeshift pres setup in Birmingham to the network feed sent round the country, so for MT it is business as usual opting their transmitters out of that feed - it wasn't the MT studio output that was being fed to the networks.
South East at that time had no opt-out ability, so network took their studio output.
Nowadays with centrally located multiplexing it's all set up completely differently.