If one puted that in a movie plot I think few would believe it.
The launch code for US nukes was 00000000 for 20 years:
http://arstechnica.com/tech-policy/2013/12/launch-code-for-u...
First it was changed to include one capital letter, one numerical digit and an asterisk.
Now it's correcthorsebatterystaple.
edit - we also used to rely on the AA's radio network - http://www.independent.co.uk/news/uk/this-britain/in-the-eve...
Are you sure you're not confusing that with nuclear submarine commanders checking that BBC Radio 4's still broadcasting as part of their "is the country still there" checks?
From http://en.wikipedia.org/wiki/Letters_of_last_resort :
According to Peter Hennessy's book The Secret State: Whitehall and the Cold War, 1945 to 1970, the process by which a Trident submarine commander would determine if the British government continues to function includes... establishing whether BBC Radio 4 continues broadcasting
But I presume that there is also a more active communication method as well. If the Prime Minister wanted to launch the nukes then he would be able to send a message to the subs and launch immediately.
I've always understood "dead man's switch" to literally mean something operating without human intervention. Probably explains the confusion!
That said, the options available to the commander do leave some room for interpretation:
- retaliate with nuclear weapons;
- not retaliate with nuclear weapons;
- use his own judgement; or
- place the submarine under allied country command - if possible
Yeah, hence my gutless hedging with "sort of".
Maybe "failsafe" would be a better term, like in the film of that name, where the bombers are instructed to ignore any order to abort once they pass a particular point. (Actually, "fail-deadly" would be more accurate: why wasn't the film called that?)