As summarized by Wikipedia:
Operation London Bridge (also known by its code phrase London Bridge Is Down) is the plan for what will happen in the United Kingdom on and immediately after the death of Queen Elizabeth II. It includes planning for the announcement of her death, the period of official mourning, and the details of her state funeral. Some critical decisions relating to the plan have been made by the Queen herself, although some can only be made by her successor, Charles, Prince of Wales, after her death.
As of 2017, the phrase “London Bridge is down” is expected to be used to communicate the death of the Queen to the prime minister of the United Kingdom and key personnel, setting the plan into motion. The plans were first created in the 1960s and are updated throughout each year, involving various government departments, the Church of England, Metropolitan Police Service, the British Armed Forces, the media, the Royal Parks, London boroughs, the Greater London Authority and Transport for London. Reporting on the preparations, The Guardian described them as “planned to the minute” with “arcane and highly specific” details.
Here is a brief video description:
The Queen’s death and funeral plans works concurrently with Operation Spring Tide, the plan for the Prince of Wales’ accession to the throne. The day after the Queen’s death, the Accession Council would meet at St James’s Palace to proclaim the new monarch. Parliament would meet that evening when MPs would swear allegiance to the new monarch and express condolences for the Queen’s death. Most parliamentary activities would then be suspended for 10 days. At 3:30 p.m., the new monarch will host the prime minister and the cabinet for an audience. Two days after the Queen’s death, proclamations of the new monarch would be made by the Scottish, Welsh and Northern Irish administrations.