[ben_w]

> (The old BBC Television Centre in London had three independent supplies, I believe)

There's a story, possibly apocryphal, that the UK nuclear deterrent submarines used the continued broadcast of Radio 4 as a dead-man switch to determine if nuclear war had broken out and they needed to open the safe containing their orders.

Which is to say: What counts as "critical national infrastructure" can be surprising.

[implements]

Radio 4 on long wave, I believe - which is only guaranteed until the end of June this year because the BBC’s stock of irreplaceable high power valves is running out. As well as triggering Armageddon the LW signal also switched older electricity meters (phew, back on topic!) between standard daytime and cheap overnight power.