Letter of Last Resort
The Royal Navy has four submarines that are each armed with nuclear payloads. If there is ever a potential nuclear exchange, these submarines will be required to release their payloads. However, the order can only come directly from the prime minister.
Given the fact that the prime minister or any other important cabinet member with the power to launch a nuclear attack is incapacitated, each submarine commander is given a sealed envelope with strict instructions as to what to do in such circumstances.