London — The strategic focus has shifted.
With the landing of British troops on the island of South Georgia, it is no longer whether Britain will need to use force but how much force will be required to regain the Falklands Islands themselves.
Before Sunday's outbreak of hostilities on the rocky former whaling station 800 miles east of the Falklands, British strategy had been to use diplomacy, backed by force, to try to regain control of both island groups.
The search for a diplomatic settlement has by no means ended. But the strategy has shifted to force backed by diplomacy.
The British military action so far has three main aims:
* To take control of South Georgia, presumably as a staging base for moving on to the Falklands (see story Page 8). This was achieved successfully Sunday.
* To stop Argentine intelligence aircraft or attack planes from harassing the British naval forces.
* To deny supplies to Argentine forces on the Falklands.
The hope in London is that gradually escalating air and naval pressure will force the Argentines to yield before a costly and difficult amphibious invasion of the Falklands has to be faced.
Decisions have been being forced on London quickly, however, by rapidly worsening winter weather in the South Atlantic, where waves are reportedly reaching heights of 40 feet.
At the same time, Prime Minister Margaret Thatcher has to be seen to have exhausted all possible diplomatic avenues. That is why she sent a message to US Secretary of State Alexander M. Haig Jr. late April 24 asking him to keep on with his mediating mission.
Mr. Haig did so when he met Argentine Foreign Minister Nicanor Costa Mendez in Washington April 25. He also discussed an Organization of American States meeting set for April 26 at which Argentina intended to invoke the Rio treaty, which calls for collective hemisphere defense against Britain.
The landing of British forces on South Georgia, including the earlier shooting incident between British helicopters and an Argentine submarine in the area, has put new pressure on Mrs. Thatcher here and at home.
Just before the British landing was announced here, Labour opposition leader Michael Foot and Liberal Party chief David Steel called for every possible effort to intensify diplomacy. Both said they supported the initial helicopter action against the submarine if it had been ''in self-defense'' as the British Defense Ministry claimed.
But both clearly reflected concern in the House of Commons that Britain may forfeit some of its hard-won public support in the United Nations Security Council, in Western Europe, and here at home by actually engaging the enemy and risking loss of life.
Both leaders, as well as Roy Jenkins of the Social Democrats, also want intelligence briefings by Mrs. Thatcher on latest developments. So far, lacking such briefings, they have stessed their freedom to criticize.
Even as the use of force began to escalate, both Britain and Argentina had appeared to yield ground in their negotiating positions.
The central sticking point is the issue of sovereignty. Britain insists it still has it, and that Argentina must recognize the point by pulling troops off the Falklands and allowing the inhabitants to say how they want to live.
Behind that position, signs are that Mrs. Thatcher is now prepared to see an Argentine flag flying over the Falklands capital of Port Stanley even during an interim government - that is, between the time Argentine troops leave and final decisions are made about the Falklands' future. The Foreign Office has indicated to British newsmen that it would be foolish to get ''hung up'' on the issue.
It is also pointed out here that a blue and white Argentine flag has flown for many years at Port Stanley airport, which Argentine planes have used on commercial flights.
For his part, Argentine Foreign Minister Costa Mendez said Sunday in New York that the attack on the submarine ''technically'' created a state of war with Britain. Later, in Washington, he told reporters that ''there is never an end to diplomacy.''
In an interview with British commercial television April 24 (before the British landing on South Georgia) and shown here April 25 he had sounded conciliatory.
The Falkland Islanders, he said, could keep their own ''religion, habits, laws. . . . If they want to leave the islands we will buy their property from them at a just price. . . . If they want to move to Argentina we will give them loans to buy new homes.'' Argentina was ready to discuss a refuelling station on the islands for British ships, and a base for Antarctic studies. Natural resources around the islands could be shared.
''We are ready to negotiate everything,'' he said, ''except sovereignty itself.''
Even though a number of politicians think that the 1,800 Falkland Islanders ought not to have a virtual veto over British foreign policy, Mrs. Thatcher insists their wishes remain paramount. Argentina has not agreed that they should have a choice. Buenos Aires wants its own flag flying and its own administrators to remain after its troops leave.
It is reported here that Mr. Haig has been trying to find a formula and language by which the islanders could be polled and to which the Argentines would not object. So far that effort has not succeeded.