Britain is moving toward the purchase of the U.S. made Trident intercontinental missile for its new submarine ballistic system to replace the present Polaris nuclear force in the 1900s.

Although a final decision is not scheduled until early next year, the modernization of Britain's independent strategic deterrent with a Trident missile and British-made nuclear submarines is now favored inside the Defense Ministry, according to infromed souces.

While insisting that the decision-making process is "not near a conclusion yet," Prime Minister Margaret Thatcher's defense secretary, Francis Pym, acknowledged today that Thatcher would discuss the subject with President Carter during her visit to the United States next month.

At the same time, Pym expressed strong support for the recently concluded Soviet-American strategic arms limitations agreement, saying "it would be a great setback if it were not ratified" by the U.S. Senate.

"i wish SALT II would be ratified soon, this week, this month or next month," he said.

Ratification, he said, would ease NATO deliberations next month on deployment of new medium-range nuclear weapons in Western Europe, which Britain supports. Pym said he regarded Soviet President Leonid Brezhnev's recent offer to negotiate curbs on nucllear arms in the European theater as a propaganda effort designed to discourage NATO's approval of production and deployment of the new missiles.

Speaking to a group of U.S. reporters, Pym said the Western media "has given too much attention" to Brezhnev's offer, adding, "The West finds itself on the defensive when it shouldn't be."

Since taking office in May, Thatcher's Conservative government has devoted much money and emphasis to modernizing and improving the morale of the British military against a backdrop of tough talk about the growing Soviet threat to Western Europe.

Thatcher imediately gave generous pay raises to the 325,000 members of the armed forces and then decided to increase total defense spending next year by the entire 3 percent agreed on by NATO countries in 1977. Britian is doing so despite its ailing economy and the deep cuts being made by Thatcher in all other government spending except crime control.

Thatcher's government also is a leading proponent of modernizing NATO's medium-ranage nuclear weapons in Western Europe with 572 new U.S. made Pershing 2 and ground-launched cruise milliles, Britain has agreed to have a number of the new nuclear missiles deployed at bases in eastern England.

Pym said today that the British are optimistic that all the NATO defense ministers will agree next month to produce and deploy the missiles. The British expect that at least Italy and Belgium will agree to join West Germany in allowing the weapons to be deployed at U.S. bases on their soil. g

Pym said Britain also supports the NATO plan to use the period of several years between a dicision next month and actual deployment of the missiles to explore with the Soviet Union possibilities for mutual arms reductions in Europe.

Britain still devotes the equivalent of nearly 5 percent of its gross national product to defense, higher than any NATO country except the United States. It also is the only other NATO nation to contribute to all three NATO defense functions: strategic nuclear weapons targeted on the Soviet Union, tactical nuclear weapons for the defense of Western Europe and conventional army, navy and air forces.

There has been some doubt whether the previous Labor government would decide that maintenance of Britain's relatively small strategic nuclear deterrent was worth its great expense after the Posaris submarine fleet was retired in the 1990s. Former prime minister James Callaghan had remained noncommittal because of opposition inside the Labor party to replacing Polaris.

But since Thatcher's Conservatives came to power, the only question has been what kind of new nuclear weapon system should replace Polaris. There was some support for a cheaper, land-based cruise missile system, but the Trident appears to have won Defense Ministry favor because of greater certainty here about its development and capabilities and because of the greater maneuverability of submarines far beyond Britain's small land mass.

Under the plan now considered to the Defense Ministry, the present four nuclear-powered submarines armed with 64 Polaris missles would be replaced by five nuclear submarines armed with Tridents carrying British-made nuclear warheads.

Trident's range is about 4,000 miles, or almost double the range of Polaris.

Britain is improving its air defenses with new Tornado intercepter-fighters, being built jointly by Britain, West Germany and Itally, for deployment in 1983, a dicision made by the previous Labor government.

It also is considering the purchase of an improved U.S. version of the British Harrier jump-jet fighter powered by British-made Rolls-royce engines. Exploratory negotiations are now underway and may also be discussed by Thatcher and Carter next month.