[arethuza]

It's also fairly well documented that UK Trident sub warheads do not have Permissive Action Links - the crews of the subs do indeed have the ability to launch without receiving any codes.

The justification for this is pretty simple - timing. In the event of a launch from a likely enemy (the Soviet Union in the bad old days) there simply wasn't enough time to guarantee that a code be transmitted before weapons bursting.