Which royal yacht's decommissioning is depicted in season 5 of The Crown?

The royal yacht decommissioned in season 5 of The Crown is HMY Britannia.

Britannia was the British monarch's yacht from 1954 until 1997 and served Queen Elizabeth II on royal tours, Commonwealth visits, and private family voyages. Season 5 depicts the decision to retire the vessel during the period covered by its 1990s storyline, reflecting the expense of operating it and changing attitudes toward royal spending.

The yacht was built at the John Brown & Company's shipyard in Clydebank and launched in 1953. During its service, Britannia travelled more than a million nautical miles and visited many countries. Its decommissioning took place at Portsmouth on 11 December 1997, after the Labour government's 1997 election victory. Britannia is now preserved as a visitor attraction in Leith, Edinburgh, where it is known as the Royal Yacht Britannia.