Letters of last resort and the human decision loop

How do letters of last resort fit into Britain's submarine deterrent?

They are a contingency instruction for a submarine isolated after a catastrophic loss of national command, preserving a human decision process rather than enabling remote launch.

Normal coded communications and the letters of last resort are two distinct routes for a submarine cut off from national command. The weapons cannot be remotely fired from shore; the crew decodes an order and retains people in the decision chain. If broadcasts and command links disappear after a catastrophic attack, the captain opens the prime minister's sealed letter for guidance. The broader argument is institutional: deterrence depends on trained human judgement at critical points, even though every command arrangement carries risks.