Do you have a plan ready for the day you are deposed? Don’t be caught with your bridges down.
When the time does come, Sir Christopher Geidt, the Queen’s private secretary, will be the first official to convey the news. Geidt will call the prime minister and use the secret “The Queen is dead” code. In the past, palace officials used a code to make sure switchboard operators weren’t the first to find out. When King George VI (the last British monarch to pass) died in 1952, the code that palace workers used was “Hyde Park Corner.”Queen Elizabeth’s secret code is “London Bridge is down.” This statement will kick off Operation London Bridge, the plan to alert the world that Queen Elizabeth II has died.
Source: When Queen Elizabeth II dies, what will happen? Secret 10-day plan revealed