British warships, moving for the first time to within sight of the Falkland Islands, bombarded Argentine occupation forces there again today as officials here signaled significant shifts Britain has made in negotiations to achieve a diplomatic settlement.

With expectations increasing that the new British military action is preparation for an invasion within a week if the mediation attempt by U.N. Secretary General Javier Perez de Cuellar fails to achieve a settlement soon, British Foreign Secretary Francis Pym and other officials continued to show greater flexibility in Britain's negotiating position.

Saying Britain was willing to consider a wide range of possibilities including joint rule of the Falklands with Argentina, independence for the islands or a U.N. trusteeship, Pym told the Foreign Affairs Committee of the House of Commons today that "we have not taken the line that this is British sovereignty forever and a day. It could be, but it need not be."

Moving farther away from previous government statements about self-determination for the 1,800 inhabitants of the Falklands that appeared to give them a veto over any settlement, Pym said British negotiators "would bear in mind their wishes" after consulting them during an interim international administration of the islands following an Argentine withdrawal.

But he also repeated that "I am not prepared to exclude any possible military action that may be required in the future" if diplomacy failed to produce a negotiated settlement.

Other officials said most of the movement in the British position occurred in negotiations last week through Secretary of State Alexander M. Haig Jr. on a Peruvian government peace initiative before the diplomatic focus shifted to the United Nations. Under some pressure from Haig, according to sources here, the government of Prime Minister Margaret Thatcher dropped demands that full British administration be restored to the islands after an Argentine withdrawal and that the Falkland Islanders have a virtual veto in negotiations about the islands' future.

Instead, the Thatcher government has agreed to accept international administration of the Falklands. Britain no longer is demanding that its colonial governor be restored to authority in the Falklands, one informed source here said. But it reportedly wants all Argentine military administrators to leave and a U.N. or other international administration to operate through the islanders' legislative councils and the few remaining British civil servants headed by the colonial financial secretary.two points, according to senior government sources: A supervised Argentine withdrawal from the Falklands must be part of any cease-fire and Argentina cannot be guaranteed sovereignty over the Falklands or any other "prejudged outcome" of the negotiations about their future.

"We have been remarkably flexible considering the Argentinians invaded our territory," said one senior British official, who indicated the government would not modify its position further. "We do not see from the Argentinians the sort of flexibility we have been looking for," he said, "and there's not much more we can do about it."

Indicating the government would have to decide within a day or two whether the U.N. talks are likely to produce a settlement, another senior official said, "We can't pursue this indefinitely. We can only clarify our position so much and then you come against the crunch."

British Defense Secretary John Nott said the government would not allow Argentina to stall in the United Nations and slow Britain's military timetable.

"We have always expected that if they could they would take this into the United Nations with the idea of playing it along diplomatically," Nott said. "We have always throughout this thing moved forward remorselessly, so if we are forced and have no other choice but to repossess the islands, that will be open to us."

Analysts here said today that Britain's military forces are prepared to invade the Falklands within the next week if the government gives the order.

Nott indicated during a television interview yesterday that Britain's task force blockading the Falklands could try to pressure the Argentine forces there into surrender by isolating and harassing them for several more weeks or even months. But he and other government ministers appeared to be preparing the British public for the inevitability of a bloody invasion if a negotiated settlement were not produced soon.

Meanwhile, Britain reportedly has set up a 100-mile-radius "controlled airspace" zone around Ascension Island, which has become an important staging and supply post for its task force.

Conservative Party Chairman Cecil Parkinson, who as paymaster general is a member of Thatcher's "war cabinet" of senior ministers, said on television tonight that if an invasion becomes necessary, there is a relatively brief "time window during which we have to operate."

Analysts here pointed out that most of the 70 military and civilian ships in Britain's task force and the estimated 5,500 troops trained for landings on the Falklands are now in the South Atlantic.

Ships carrying the last 2,000 troops and 20 additional Sea Harrier fighters have left Ascension Island and are days at most from the rest of the task force.

Two British destroyers being added to the task force, one to replace the sunken Sheffield, departed only today and the liner Queen Elizabeth 2 is still being outfitted to carry 3,000 more troops to the area. But the destroyers are not believed to be needed immediately and the additional 3,000 troops have been trained for maintaining a garrison on the Falklands after their recapture.

Analysts said that once all the British invasion troops have arrived off the Falklands, the "time window" to which Parkinson referred could be only a few days or weeks.

Their combat readiness, the onset of winter in the South Atlantic and the vulnerability of the greatly increased number of British ships in the war zone to attack by Argentine aircraft and submarines dictate landing attempts as soon as possible, according to the analysts.

One of Britain's leading military historians, Oxford University Prof. Michael Howard, wrote in yesterday's Sunday Times that "the longer we wait, the more rapidly will our prospects, either of maintaining a successful blockade or mounting an invasion, continue to erode."

The resumption of British military attacks on the Argentine forces on the Falklands yesterday and today--after a four-day lull during foggy weather in the area and intense diplomatic activity at the United Nations--appeared designed both to make the blockade more successful and to prepare for an invasion.

According to British correspondents with the task force a number of destroyers and frigates left the rest of the force to move in close to East Falkland island to prevent blockade running by Argentine ships and planes, amid indications some may have broken through during foggy weather last week. The correspondents said the ships are now in sight of the Argentine forces and the inhabitants of East Falkland.

This exposes the ships to greater danger of attack by Argentina warplanes flying from mainland bases, but it also enables them to police the blockade more closely.

Britain's predawn naval bombardment of Argentine positions yesterday and today was described here as a "softening up" of the Argentine defenders for later landings by keeping them on battle alert and preventing them from knowing when an invasion actually is beginning.

Informed that the Argentine government had first reported yesterday's attack as a British invasion, a Navy briefing officer aboard the aircraft carrier Invincible told British correspondents, "That is just what we were hoping for. It means that we have scared the hell out of the."