UK nuclear weapons are designed first and foremost to be secure and safe. A rigorous system of processes ensures the safety and thoroughness of the operating system for the UK nuclear deterrent.
The fusing, arming and firing circuits are designed such that the warhead must have experienced missile launch and ballistic deployment before it can detonate. Launching a Trident missile from a submarine is a complex activity. Prior to launch, the command and control structure on board the submarine would need to be satisfied that the Prime Minister has issued instructions to launch nuclear weapons. A coordinated effort involving key individuals from the boat's company of 150 is required to launch the missile. The number of participants required to act in concert means that the 'Permissive Action Link' type safeguards found in other systems are not relevant in the SSBN domain.