Britain's dramatic victory in the Falkland Islands has touched off a major debate here over British defense policy that could have profound impact on the nation's military role in the Western alliance.

There is already a movement in military circles to delay or to scrap Britain's planned purchase of the U.S. Trident II submarine-launched, long-range nuclear missile system, which the Thatcher government announced in March.

Several defense specialists are arguing that the $13.5 billion for Trident, which some analysts predict could rise to nearly $18 billion during the next decade, might better be spent to upgrade British conventional forces. Members of Britain's powerful Navy lobby are calling for major spending increases for the Royal Navy, which performed so successfully in the South Atlantic but which presently is slated for significant cutbacks in the size of its fleet.

While those involved in the debate insist they foresee no drop-off in London's commitment to the North Atlantic Treaty Organization, there is renewed emphasis here on what are called "out of area" problems--Britain's interests and defense obligations outside of Europe and NATO.

"We must have the capacity to act independently," Prime Minister Margaret Thatcher told Parliament this week. "We need both the power to act and the will to see it through."

Inevitably, some analysts say, budget restrictions will force London to choose among maintaining its independent nuclear deterrent, its present large role in NATO and its commitments to far-flung territories such as the Falklands.

The early focus of the public debate is on the fate of Defense Secretary John Nott, the architect of the Trident II and naval cutback proposals. Naval advocates say that if Nott's plan had been in effect at the time of the Falklands crisis, the Navy would have been incapable of mounting the task force that was sent to reclaim the islands.

A year from now, for example, under current plans, the Navy will have only one aircraft carrier available--a new one--because the government plans to sell the Invincible to Australia and the Hermes is due for a long and elaborate refitting in drydock. The two assault ships, which played key roles in landing troops on East Falkland Island, had also been slated for mothballs until pressure from the Navy forced the government to grant them a reprieve.

Altogether, Nott's projections call for the Royal Navy to consist of only 42 ships by 1985. The admirals and their allies, including an influential group of Conservative politicians and shipbuilding interests, say they need at least 50 ships, not including replacements for the four vessels lost in the Falklands war and the five or so ships they believe will have to be stationed around the islands for future defense.

Nott has insisted he will not step down and that there will be no major changes in defense policy. Others are less certain.

"He certainly looks shaky," said one defense analyst. "The question is whether he has the confidence of the prime minister, and it doesn't appear that he does."

Nott barely survived in April, when he was blamed, along with the foreign secretary, Lord Carrington, for being caught unprepared by Argentina's invasion of the islands. Carrington was allowed to resign, and sources said Nott offered to quit but Thatcher insisted he stay on. While the prime minister says publicly that she still supports him, many observers think she is only waiting for a decent interval to pass before she accepts his resignation.

Despite the undeniable success of Britain's mission to the South Atlantic, many analysts believe the military lessons of the Falklands are less than clear. While its advocates emphasize Royal Navy achievements, skeptics point out that it took only one Exocet missile to sink a destroyer and that World War II-vintage bombs from obsolete Canberra bombers did major damage to other vessels.

"The Navy was hard hit by Mr. Nott's cutbacks and they are clearly attempting to take advantage of the political situation following the Falklands to advance their cause," said Col. Jonathan Alford of the International Institute for Strategic Studies here. "But there are equally sound arguments not to build vulnerable surface ships and I don't think we'll see, or should we see, major changes in the size and shape of the Navy because of the Falkland Islands."

Trident, however, is a different matter, according to Alford and other analysts. While Nott has pointed out that the Trident system will be purchased in increments and cost less than $1 billion per year even during peak outlays, others expect the price tag to rise.

"Payments for the Trident system will swallow between 15 and 20 percent of the Ministry of Defense's capital expenditure from the end of this decade until the middle of the next," writes David Greenwood, director of the Center for Defense Studies at Aberdeen University, in an article published yesterday in Defense Attache magazine.

He concludes, "As time goes by, Trident will look like a less and less attractive proposition. Renewed debate over posture and priorities will be inescapable if this occurs, and the Falklands experience will be influential, though not decisive, in that argument."

As for NATO, the Falklands episode already has had immediate impact by siphoning off a large segment of the British fleet, normally used for North Atlantic and North Sea duties. The question now is whether a continued British naval presence around the islands will cause a longer-term weakening of NATO's sea defenses.

Ministry of Defense officials here say it will not. But Royal Navy advocates, again mustering arguments for more ships, insist Britain cannot maintain its NATO commitment and keep a permanent floating garrison in the South Atlantic.

Besides allowing for more ships, the money saved from scrapping Trident could help provide new military hardware, including the sophisticated radar that could have warned British ships of aerial attack. Even Nott, in a press conference this week, agreed that "clearly it would have been a help to us to have an early airborne-warning system." But, he underlined, the issue is how to pay for it.