With this year's sales already double the 1984 total at about $6 billion, Britain's defense industry has launched a major bid to supply the U.S. Army with an antiaircraft missile to replace its recently canceled Division Air Defense antiaircraft gun system (Divad).

The British offering, called Rapier, claims to be everything Divad was not -- a combat-proven, in-use weapon that combines missiles, radar and its own armored vehicle and can be delivered on time with no cost overruns. Its manufacturer, British Aerospace, says Rapier will sell itself.

To help it along, however, the British government is prepared to put considerable muscle behind the bid. If need be, Defense Ministry officials say, they would not hesitate to enlist the aid of Prime Minister Margaret Thatcher to point out to the Reagan administration the political, as well as economic and military, advantages of buying British.

During the past year, Thatcher has appealed personally to Reagan, Saudi Arabia's King Fahd and Indian Prime Minister Rajiv Gandhi to sign defense contracts with British industry. Her sales pitches have ranged from reminders of Britain's status as a "loyal" ally to offers to stretch financing terms, invest British money in the purchasing country and take part payment in goods.

"In the world of today," Defense Secretary Michael Heseltine said recently, "any major military or civilian contract, with any country, has escalated decision-making to the highest political level. No one puts more endeavor into fighting that competitive political battle than our prime minister."

British Aerospace corporate sales director Alec Sanson says the Thatcher government has been "more and more helpful in providing political support" and diplomatic lobbying for deals such as its recent $5 billion aircraft sale to Saudi Arabia. "Without it, we can't succeed," Sanson said in an interview.

This week, Rapier was on display at a U.S. Army weapons show in Washington and at the Pentagon. The model is on loan from the British Army of the Rhine in West Germany, which flew it over and is demonstrating its virtues.

In recent weeks, a team from British Aerospace accompanied by the Defense Ministry's director of military guided weapons has visited the Pentagon.

Officials here expect the short list of bidders being considered for the multibillion- dollar contract to replace Divad to be completed by the end of November, with evaluations next spring and a decision by early summer.

Despite anticipated competition from a Swiss model, called ADATS, and the Franco-German Roland, Britain seems confident that Rapier -- whose stationary version was chosen last year by the U.S. Air Force to protect cruise missile installations in England -- will prevail. Monday, British Aerospace announced that the U.S. Air Force had placed an order for more stationary Rapier systems to protect two NATO bases in Turkey.

Government and industry officials here agree that the high-powered sales efforts are part of an arms export strategy that has combined the marketing and planning resources of both sectors as never before in Britain. It has infuriated Britain's competitors, particularly France, which despite an international reputation of its own for government and industry sales cooperation, says that Thatcher has gone too far in pushing for advantage in the world marketplace.

Curiously, the use of such influence under Thatcher has increased in near inverse proportion to her domestic efforts to separate government from private industry. Thatcher's economic policies are based on her strong belief that Britain's long-term economic recovery depends on the ability of British business to stand on its own two feet, or fail through lack of competitiveness and privately nurtured profitability.

As part of an overall state divestiture program, the government this year sold its remaining 48 shares of British Aerospace, the country's premier defense contractor. Only the Royal Ordnance factories remain under state ownership, and they will be on the auction block soon.

Along with the diminishing of direct state involvement in arms production has come the demise of the Defense Sales Organization within the Defense Ministry. Its beefed-up replacement is the Defense Export Services Organization, whose job is to help private British defense contractors sell abroad.

At the department's head is the former marketing director of British Aerospace, Colin Chander, who is one of a growing number of industry executives on loan to the ministry. The department maintains its own offices in potential sales capitals around the world, including inside the British Embassy in Washington.

The overlaps are designed to "put Her Majesty's Government's seal of approval" on export sales, another British Aerospace executive said.

A major ministry reorganization begun by Heseltine early this year ran into criticism when Peter Levene, chief executive of United Scientific Holdings, a major supplier to Britain's own armed forces, was named head of the defense procurement office. After a parliamentary inquiry to examine possible conflicts of interest, Levene took the job.

Defense and industry officials argue that such practices benefit everyone concerned, despite their seeming contradiction of Thatcher's economic dogma. By getting the government out of businesses such as weapons production, it was argued, business could streamline procedures and cut unnecessary costs without paying a political price.

Last month's sale to Saudi Arabia of 72 Tornado jet fighters will allow British Aerospace to breach a production line gap of several years between filling the last orders for Tornados by Britain's own Royal Air Force and starting production in the 1990s of a new European jet fighter.

Most important for the government, in the view of Thatcher's political advisers, the arms deals mean that thousands of British workers avoid being added to already lengthy unemployment rolls. The Saudi contract saved as many as 30,000 jobs, according to British Aerospace.

The employment of the prime minister for the sales pitch is something to be used sparingly, a senior Defense Ministry official said. But when called upon, he said, Thatcher has not hesitated.

"We just go to her and say, 'We need you to step in.' A number of suggestions go up, giving talking points like the advantages in the quality of the equipment, or the fact that U.K. forces are better. Then we suggest the political arguments, like in the case of France for the United States," he said. "It wouldn't take much to conjecture that we are a better ally."