British Foreign Secretary Francis Pym will fly to Washington on Thursday with British alternatives to unacceptable parts of Argentina's detailed new proposals for a peaceful resolution of the crisis over the Falkland Islands, Prime Minister Margaret Thatcher told Parliament today.

Although the Argentine proposals "fall short in some important respects" of what she and Parliament have been seeking, Thatcher said, "I regard this as a stage in the negotiating process which now must be continued. We are examining the proposals very closely. We shall seek to put forward our own proposals."

Stressing that "we remain committed to seeking a diplomatic solution if at all possible," Thatcher refused to discuss details of the Argentine proposals, which have been described here as lengthy and complex. But she said "among the many problems the Argentine proposals present is that they fail to provide that the Falkland islanders should be able to determine their own destiny, and the British Parliament has always said the wishes of the islanders are paramount."

Argentina has reportedly offered to withdraw its military forces from the Falklands, provided it shares interim administration of the islands with Britain under U.S. supervision and its sovereignty claim is negotiated to its satisfaction by the end of this year. The proposals apparently do not offer the islanders any choice over the Falklands' future, which Thatcher has made an important point of principle for Britain.

Thatcher made no direct reference in Parliament to the question of shared British and Argentine interim administration of the islands under the flags of Britain, Argentina and the United States, which many members of her own Conservative Party strongly oppose.

Asked by one Conservative member if she would consider the Argentine proposals "with great caution, bearing in mind that if an aggressor is even half compensated for his aggression that will be encouragement to others," Thatcher said, "I take your point. It has been made very, very strongly, and I believe it has been felt on all sides of this house."

Pym later told reporters after an evening Cabinet meeting that shared British and Argentine interim administration of the Falklands is "a different point" than the prior Argentine military withdrawal Britain has demanded.

British sources said that, although the Argentine proposals represent from the Thatcher government's point of view a big step backwards from what Haig had taken to Buenos Aires after detailed talks in London, they contain some changes in Argentina's negotiating position that could eventually lead to a break in the diplomatic impasse. "We are not sweeping the proposals off the table and trying to start from scratch," one source said.

British sources said Pym will explore in Washington whether further Argentine movement appears possible in what is described here as "the environment of pressure" on Buenos Aires produced by international sanctions and the military threat of the powerful British fleet approaching the South Atlantic. Pym, in turn, is expected to learn from Haig and other officials and politicians in Washington how far the United States will urge Britain to move diplomatically to gain a settlement.

After Thatcher discussed with her Cabinet tonight the British response to the Argentine proposals, Pym told reporters, "Negotiations continue. It is difficult, but we are doing our very best to achieve a diplomatic solution."

Meanwhile, Thatcher told Parliament the British naval task force "continues steadily on its way" toward the South Atlantic. Amid rumors here and in Washington about the progress of the naval force, it was reported here to be a week or less from the vicinity of the Falklands. But its position can only be guessed at from politicians' statements and censored reports from British journalists aboard some of the warships.

British Defense Ministry officials and diplomats refuse to disclose the location of the aircraft carriers, destroyers or other warships leading the task force. They also refused today to comment on a Boston Globe report that several of them had left the main body of the fleet to head for the island of South Georgia, a dependency of the Falklands about 800 miles east of them which Argentine forces invaded the day after they seized the Falklands capital of Port Stanley on April 2.

In Washington, U.S. specialists described the British fleet as spread out from the Mediterranean to Ascencion Island, with April 28 as probably the earliest date by which substantial elements could reach the Falklands area. The faster attack ships are unlikely to outpace the supply ships on which they are dependent, said these sources. They added that while some lead elements might already have headed for South Georgia, this was not yet clear.

It was speculated in Buenos Aires that the flotilla was zig-zagging to slow its arrival but that by Thursday it should reach the point of decision on whether some ships should break off for South Georgia. It was assumed in these Argentine accounts that all ships had already passed Ascension Island.

Some well-placed sources here pointed out that retaking South Georgia, which is not nearly so heavily defended by the Argentines as East Falkland Island, on which Port Stanley is located, would be a logical early objective of the British forces. They also said that military commanders were concerned that the battle readiness of the 2,000 or more British marines aboard warships in the vanguard of the task force could deteriorate rapidly if they did not move onto land after a month or so at sea.

Some members of Parliament also have said that retaking South Georgia with a minimum of casualties and raising the British flag could give Britain an important boost in morale and increase psychological pressure on the Argentine government. But some diplomatic sources here cautioned that any hostilities could set back negotiations, jeopardize the support Britain now has in Europe and elsewhere, and increase Latin American sympathy for Argentina.

The 22-year-old commanding officer of 22 Royal Marines who resisted the seizure of South Georgia on April 3 said today that the marines killed 10 to 15 Argentines, wounded at least 20 others, shot down two Argentine helicopters and damaged a ship before finally surrendering to overwhelming Argentine forces.

"We had forced Argentina to take South Georgia by military action," Lt. Keith Mills said at a press conference following the marines' return by the Argentines to Britain via Uruguay. "They could not say they walked into South Georgia and captured it without resistance. We made sure it was a separate political issue.