QUEEN Elizabeth II calls it ''our yacht.'' You or I might prefer to describe the 412-foot Britannia as an ocean liner. Whatever it is, the British monarch's floating palace is on the block. While socialists are cheering the government's move, many supporters of the ruling Conservative Party see it as close to lese-majeste, a crime against the sovereign. Barry Field, a Conservative member of Parliament, says: ''The queen without Britannia is like the Tower of London without the crown jewels.'' Prince Edward, the queen's youngest son, has come to the defense of Britannia, which Elizabeth named and launched days before her coronation in 1953. ''No other country in the world has anything to match Britannia,'' he said on hearing the news about the ship's fate. ''She is a symbol of Britain.'' But Prime Minister John Major, justifying the decision to sell Britannia, says the ship is just too expensive. It needs a refit costing $28 million. Running costs alone amount to $19 million a year. Much of the money is spent on a crew of 250 who wear plimsolls lest their feet make a noise on the decks. They give and receive orders via hand signals for the same reason. News that the vessel - in which Queen Elizabeth has traveled a million miles and visited 135 countries - was up for sale has awakened worldwide interest among potential buyers. BUT the Ministry of Defense, which funds Britannia, says the ship will not be allowed to end its days as a nightclub on the California coast or sailing the high seas as a real-life love boat owned by a giant corporation. Foreign ownership is not allowed, and sales information issued by the Cabinet Office in London starchily stipulates: ''All offers should take into account the need to be sensitive to the royal heritage.'' Government officials say they would like Britannia to end up as a conference center or as a tourist attraction at a British port. For the masses, there would be plenty at which to gape: The ship has a reception room for up to 250 guests and two principal suites, with elevators and sitting rooms 25 feet long. Most of Britannia's furniture and fittings would be part of the sale. These include 56 18th-century Hepplewhite dining chairs, a sword in a gold-and-jeweled scabbard, several valuable oil paintings (one of Lord Nelson), and a dining table centerpiece consisting of two camels and palm trees with ruby dates. Britannia is estimated to fetch $50 million to nearly $250 million. Still, the queen won't have a yacht anymore, which alarms leading figures in London's financial district. Britannia has often been used as a showcase for British industry. Michael Cassidy, chairman of the City of London Corporation's policy committee, says: ''We have found the present royal yacht to be a remarkable marketing tool when moored off Hong Kong, Manhattan, or Bombay. Local businessmen feel extremely honored to be invited on board.'' Captains of British industry are seeking the money to build a replacement. Meantime, as one columnist has put it, it's a case of a ''Floating Palace Up for Sale: Only One Careful Owner.''