London — If British forces land on the Falkland Islands, as is now widely expected, the stage will be set for what informed sources in London see as a protracted military confrontation. It could last for several weeks or months.
British strategy would be to escalate the pressure on Argentina step by step, first seizing part of the West Falkland Island, then if necessary a section of the east island, but staying away from the heavily defended east island capital of Port Stanley.
After each seizure, Britain would wait and threaten another, in the hope that Buenos Aires would yield, take off its occupying forces, and agree to talks.
Sources here see Argentina replying by using its considerable air power - some 20 French-built Mirage III supersonic interceptors, 25 Israeli Dagger fighters, and 68 subsonic US-built Skyhawk naval bombers - against British positions.
Ships from the Argentine Navy could also steam north to try to intercept British reinforcements - Harrier jump jets, Chinook helicopters, and more marines and paratroopers - on a chain of requisitioned civilian ships and on the assault craft HMS Intrepid, due to reach the war zone in about 10 days.
''The Argentinians won't give in,'' predicts one source, Robert Elliott, information officer of the International Institute for Strategic Studies here in London. ''We are looking at a long war.''
One likely result is that the peace mission of Secretary of State Alexander M. Haig Jr. may either be suspended or ended. And, as Prime Minister Margaret Thatcher predicted on television here late April 26, the British will expect President Reagan to join Western Europe and impose harsh economic sanctions against Argentina.
Thus the US will feel obliged to support its closest NATO ally against an influential Latin American country it once looked at hopefully for possible support against leftists in Central America.
One long-term result could be that Argentina will accelerate its drive to possess nuclear weapons, as a way of compensating for the antagonism of Britain, Western Europe, and the US.
With each day that goes by, Prime Minister Thatcher sounds more resolute. She rejected April 27 an impassioned appeal by opposition Labour leader Michael Foot to go back to the United Nations. Secretary-General Javier Perez de Cuellar, has appealed for talks. Mr. Foot's tone and the mood of the House of Commons April 27 seemed to signal the end of bipartisan unity as more hostilities approached.
Informed sources here sketched the following battle scenario in the coming weeks, acknowledging that the situation appeared grave.
The British task force cannot hope to attack Port Stanley itself, since Argentina is defending it with four battalions of troops totaling about 3,600 men.
Attackers would need a superiority of 3 to 1, or almost 11,000 troops. But the task force at the moment has only 1,300 Royal Marines. Another 900 are on the high seas sailing from Britain.
Thus it makes sense for Britain to take territory on West Falkland - perhaps the town of Fox Bay on the west coast of the east island, which is defended by only 900 Argentine troops. The town can be reached only across a sound and cannot be easily resupplied from Port Stanley.
By setting up radar on the northern heights of the west island, the British could see anything approaching by sea or air. Already, British long-range Nimrod reconnaissance aircraft keep a close watch on Argentine naval vessels. The three or more British submarines in the area form an effective screen against Argentine naval resupply, even though they cannot enter the sound itself, which is too shallow.
If Argentina still refused to talk, the British could take Byron Sound to the north. The west island was never occupied by Spanish forces and the British have the same political and diplomatic claim to it as to South Georgia, already retaken.
If Argentina still refused to talk, the task force could try to capture more ground, bit by bit. Against expected attacks from Argentine jets based on the mainland, the British forces could reply with 20 Harrier jump jets based at sea, and Sea King helicopters.
One immediate objective of the Harriers would be to bomb the Port Stanley airfield to try to stop Argentina from resupplying the Falklands by air.
Argentine air power would be reinforced by 11 Skyhawks based on the aircraft carrier ''25 de Mayo,'' and nine Canberra bombers.
The British have surface-to-air missiles to fire at aircraft. These missiles include the Sea Dart (range 25 kilometers or 15.5 miles), the Sea Slug (15 kilometers or 9.3 miles), and two other shorter-range missiles. The British task force, with its submarines, attack ships, Nimrod aircraft, and Harriers is a formidable force quite capable of sinking the ''25 de Mayo'' and any other Argentine ship it encounters.
But the British must contend with appalling winter weather and mountainous seas, and a supply line stretching 8,000 miles back to Britain. Argentina would be fighting much closer to home.
British troops are professionals and highly trained, but President Galtieri of Argentina is considered certain to put up a fight. Trade sanctions, even by the US, would take time to bite, and the Soviet Union might well offer compensatory help.
To prevent such a long-drawn-out confrontation, opposition politicians are calling for new efforts by Mrs. Thatcher to reach a diplomatic solution by using the UN.
But Mrs. Thatcher insists that the Argentine ploy is to keep up the impression of negotiations while British ships and men endured ''very terrible weather, gales, and freezing.'' She added that Britain ''shall always use minimum force. . . .''