Letters of last resort
These are letters written by the current British Prime Minister that give instructions containing four options to the commanding officers of the four British ballistic missile submarines in the event of a nuclear attack against the United Kingdom.
Apparently one of the indicators that a nuclear attack has happened is that the BBC's radio Four stops broadcasting. This caused an alert in 2004 when the radio station went off the air for 15 minutes.
The Four Options
1. Retaliate with nuclear weapons.
2. Not retaliate.
3. Use his own judgement
4. Place the submarine under an allied country's command.