It's certainly true that the UK has a sort of dead-man's-switch (depending on exactly what the letter of last resort says). But I presume that there is also a

It's certainly true that the UK has a sort of dead-man's-switch (depending on exactly what the letter of last resort says). 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.