Not really a confusion: if the letter of last resort (sealed orders to be opened in the event the Government is destroyed) says "launch the nukes", you do have a sort of dead man's switch, assuming the sub's captain follows the orders.
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?)
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.