Right, but the actual mechanism of the secret-message-in-a-safe-in-a-safe makes no difference to the well-known game theory of nuclear retaliation. That is what makes it an odd practice. No other country does this because it serves no purpose.

Perhaps not - but I rather like the idea of four personal hand written notes in our four missile subs, which are hopefully never opened and destroyed when the PM changes.

I rather hope they say "Don't fire your missiles at anyone, you will just make things worse".