Britain made significant moves this week to prepare for the possibility of drawn-out military confrontation with Argentina over the Falkland Islands if the shuttle diplomacy of Secretary of State Alexander M. Haig Jr. fails to achieve a settlement.

Steps to reinforce the already large naval task force now nearing the Falklands appear designed to extend its effective operation from weeks to months, to increase its military options, to provide replacements for men or materiel lost in combat and to cope with the harsh winter soon to begin in the South Atlantic.

The recommissioning of a mothballed assault ship to carry several hundred more troops, their landing craft and helicopters, the outfitting of a civilian cargo ship to carry about 20 more Harrier vertical-takeoff fighters and the requisitioning of five fishing trawlers to be used as minesweepers were announced as "additional measures to provide extra capability for the task force over an extended period."

This increases the officially announced elements of the task force to more than 50 vessels: two aircraft carriers, two assault ships, five destroyers, nine frigates, five landing ships, 16 tankers and support ships, two converted civilian cruise ships, two commercial container ships, the five trawlers, three tugs and three supply ships.

They will be carrying up to 40 Harriers, 40 to 50 helicopters, eight landing craft, military vehicles and weapons and several thousand troops. Nimrod reconnaissance planes are being used for surveillance of the South Atlantic and up to four nuclear-powered submarines are believed to be patrolling underwater.

More men, equipment and fuel are being held in reserve on Ascension Island, about halfway between Britain and the Falklands, where the vanguard of the British fleet was seen passing this week. Official secrecy now shrouds the warships' precise locations and timetable.

It has been estimated that the cost of just getting the British fleet to the South Atlantic is about $100 million. But Prime Minister Margaret Thatcher's chief secretary to the treasury, Leon Brittan, told a conference of businessmen at Oxford University last night that "there is no cash ceiling on the cost of the operation. The needs of the task force must and will come first."

Brittan said the cost so far represented a very small part of this year's British defense budget of about $25 billion. If it rose significantly, he said, it would be met "in ways consistent with the government's economic strategy." Earlier, he had said taxes would be raised, if necessary, to keep the crisis from increasing the budget deficit.

Other senior officials said they thought the government's contingency fund could cover the cost of sending the fleet to the South Atlantic while Britain negotiated an Argentine withdrawal from the Falklands. But combat or an extended stay in an inhospitable climate not far from Antarctica would be considerably more costly and no one will publicly discuss that contingency.

Human, rather than financial, costs appear to weigh much more heavily as Thatcher decides what orders to give the task force when it arrives. Politicians, pollsters and analysts have been trying to calculate how far Thatcher could go militarily without losing the all-party support that Parliament and public opinion polls have given her strategy of diplomatic, economic and military pressure on Argentina.

Public opinion polls and politicians' soundings among constituents indicate very high support for the threat of military force and for such strikes against the Argentines as sinking their ships. But that support drops off markedly when people are asked about military operations such as an assault on the Falklands, now heavily reinforced and fortified by the Argentines, that could cost heavy British casualties.

"Everyone wants an easy military victory without loss of British blood," said one Conservative member of Parliament, "which is a rather difficult tight-rope act."

Asked earlier this week if he thought the British "have the stomach for the sacrifices and casualties that might be involved" in military action, Foreign Minister Francis Pym said, "I think they must be convinced that it's not been rashly done, or ill-advisedly done, or unnecessarily done. If they have confidence in us and the way we have managed it, I think they will."

Depending on how the Argentine Navy responds to the entry of the British fleet into waters off Argentina's coast, analysts here expect British warships to try to operate initially outside the range of Argentine fighter planes. Use of surface ships to replace the submarines in enforcing a 200-mile quarantine around the Falklands near the Argentine coast would risk attacks by those planes.

But if the British fleet draws the Argentine Navy out to sea, the submarines could be used to prevent them from returning to base. This could force a primarily naval battle in which the British fleet would be superior in numbers, firepower, electronics and experience.

Speculation by analysts here on the British fleet's first move centers on an attack on the windswept, mountainous island of South Georgia, a dependency of the Falklands 800 miles to the east where the crisis began with an unauthorized landing by Argentine scrap dealers. South Georgia is occupied by a relatively small Argentine force and lies well outside the range of Argentine fighter planes.

Analysts and politicians believe this makes South Georgia an easy target for a quick British military success, raising the Union Jack once again over one part of the territory seized by Argentina. Asked about the effect of this on political support for Thatcher if the use of force became necessary, one influential senior Conservative member of Parliament said, "Retaking South Georgia would give this country a real boost."

It would also give the British task force an operating base significantly closer to the Falklands than Ascension, which is 3,500 miles to the north. South Georgia has numerous inlets in which warships could anchor but it is still 800 miles from the Falklands and will be very cold and windy in just a few weeks.

Reports from Buenos Aires had suggested that Argentine forces were lengthening the runway of the airport at Port Stanley, the Falklands' capital, on the eastern of the group's two main islands. This could enable larger planes and longer-range jet fighters fully loaded with bombs to use the field.

But the British Defense Ministry said today that two Royal Air Force technicians, who were allowed by the Argentine forces to remain in Port Stanley after the invasion, have reported that the runway there was not extended and remains only about 4,000 feet long, still suitable for some Argentine jet fighters. The primary change, according to the Defense Ministry, was the construction of parking spaces for more aircraft.

The technicians, who were flown out of the Falklands by the Argentines only this week, watched Argentine troops working at the airport and took photographs. Their information was "invaluable," the ministry said, and "not only important in itself, but also illustrative of some of the wrong information which is being circulated."

The British Defense Ministry has accused Argentina of spreading false information about the extent of its military buildup, particularly on the two main Falkland Islands. But many politicians and analysts here still believe that a decision to storm them, where up to 9,000 heavily armed Argentine troops are reported to be dug in, would be made only as a last resort.

The alternative, they added, could be a long naval blockade, which would present increasing logistical problems so far from Britain, especially as the weather worsens, and jeopardize the morale of seamen and troops aboard the ships. The longer it dragged on, the more opportunity there would be for support of the government's strategy to erode here, although British officials insist time is on their side as pressure increases on Argentina.

Questioning that judgment, one well-informed senior foreign government analyst said, "If they wind up just sailing around and around for months on end, with boredom and the weather taking their toll, this could turn out to be a disaster for Britain.