n a major review of defense policy, Britain asserted today that its recent "splendid successes" in the South Atlantic cannot "obscure the fact that the main threat" to the country remains the growing strength of the Soviet Union and its Warsaw Pact allies.

As the centerpiece for meeting that threat in the years ahead, Defense Secretary John Nott strongly reaffirmed the government's intention to go ahead with the controversial $13.5 billion purchase from the United States of the Trident II submarine-launched, long-range nuclear missile system. The British victory in the Falklands, which required sending a large naval armada to the region, has fueled the ongoing debate here over whether the Trident should be acquired at the price of further cuts in conventional forces, particularly the Navy.

Nott's comments came as he released the government's equivalent of the Pentagon's annual defense posture statement. The document was scheduled for publication in April but held up because of the Falklands. Intentionally, no revisions were made to take account of the South Atlantic battle so that Nott could assert today that the government defense priorities remained the same, principally to maintain a credible nuclear deterrent as Britain's major contribution to NATO.

Responding to critics who have cited the role of conventional forces in the Falklands in calls for the scrapping of the Trident plan, Nott said in a statement: "Much valuable experience has been gained from the Falklands conflict, but we should not rush into premature conclusions based on the dimly perceived lessons of the past few weeks." He promised that after "mature study and reflection" any changes in defense policy required in light of the Falklands experience would be proposed toward the end of the year.

Then in response to questions from reporters, Nott said that, in any case, he remains totally committed to the government's nuclear modernization plans. "The Trident program goes ahead exactly as announced," he said.

The Labor Party's defense spokesman in Parliament, John Silkin, took exactly the opposite view, declaring, "You have got to get rid of Trident" on grounds of high cost and limited strategic usefulness. He said Britain should concentrate on its newly demonstrated skills in projecting naval power.

Despite the firmness of Nott's line, there seems little doubt that some changes in defense policy will be made in the Falklands aftermath. For instance, plans to sell the carrier Invincible, flagship of the Royal Navy, to Australia, and retire the carrier Hermes are almost bound to be reversed as a result of their role in the Falklands. Nott said as much in citing Australia's offer to Britain to "reconsider" the purchase.

The burden of Nott's remarks, however, was that should examination of the Falklands success lead to restoration of some earlier cuts in the conventional fleet, other elements in Britain's defense will have to be trimmed--but not the nuclear forces.

Nott's own future is in some doubt. He nearly resigned when Argentina invaded the Falklands in April, quickly overwhelming British forces there, and his handling of the Defense Ministry has drawn criticism from some quarters even within his own Conservative Party. Should he be ousted, the chances would increase of some Falklands-related revisions in defense strategy.

Nevertheless, Prime Minister Margaret Thatcher apparently made no effort to hold up today's defense document. Speaking of the planned assessment of the South Atlantic war, Nott said, "I intend to publish a White Paper toward the end of this year, when there has been more time to study all the facts."

Release of the defense posture statement was another indication that Britain is returning to normal after its 2 1/2-month-long preoccupation with the Falklands crisis. Today's newspapers were dominated by the birth of a son to Prince Charles and his wife Diana, princess of Wales. Unemployment figures climbed above 3 million workers, a reminder that the economy is still in trouble. And London traffic was paralyzed by a subway strike.