Britain's mood is confident

''The days of the occupying Argentine garrison are numbered.''

That confident statement by British Defense Secretary John Nott in the House of Commons May 24 sums up the increasingly ebullient mood of the Thatcher government here as British forces prepare for an all-out attack on Port Stanley and the ousting of all Argentine forces from the Falkland Islands.

Diplomacy, for the moment, is not being considered. Suggestions for a cease-fire or a truce are brushed aside. London does not believe that it can negotiate with the Argentine junta now until it controls the islands.

Reporters here are being told that British settlement terms have toughened. Instead of being prepared to accept a United Nations interim force, Britain now plans to reinstate direct control in the person of Governor Rex Hunt, and to see the full sovereign status of the Falklands restored as it was before the Argentine occupation.

The Prime Minister is fully prepared to maintain a garrison on the island big enough to prevent future Argentine attacks, officials say. It is believed that the garrison will be formed by the 3,000 troops now on board the liner Queen Elizabeth II, which is being kept away from Argentine air attacks by land-based jets.

London was heartened by the May 24 decision of seven members of the European Community (excluding Ireland and Italy) to continue trade sanctions against Argentina indefinitely.

Defense officials here are careful to stress that the battle is not yet won. The Falklands terrain is difficult and the climate hostile, as Mr. Nott also pointed out.

Argentina still has three small submarines which can sink British ships and about 150 aircraft, greatly outnumbering British Harriers.

But the British government says that Argentina is sustaining large losses and that the morale and the supplies of Argentine forces on the Falklands are both low.

According to Mr. Nott, the Argentine Air Force lost 30 percent of its Air Force during the initial British landing, and two-thirds of the attacking force used in the next wave.

Clouding British optimism somewhat is a rising casualty rate of its own.

With the latest announcement here of the British frigate Antelope's crew abandoning ship May 24, and the loss of another Harrier pilot in an accident during launch, Britain at this writing had lost at least 74 men killed with another 80 or more wounded. Britain had lost two other ships, five Harrier jets and nine helicopters since the task force arrived.

This is far less than Argentina, British figures say. Argentine losses are put here at 382 deaths, plus 18 Mirage and Dagger fighters, nine Skyhawks, one Canberra bomber, three Pucara ground-strike aircraft and five helicopters. (In an attack May 24, seven Argentine jets (early reports said they were Mirages) were said by the British defense ministry to have been shot down, Reuters reports.) Another 11 aircraft were destroyed on the ground by commandos at Pebble Island. In all 52 Argentine aircraft of all types are said to have been lost.

At least until now the Labour and Social Democratic Party opposition leaders in the House support government strategy, although they insist that the door to a negotiated settlement must not be closed.

Labour Defense spokesman John Silkin asked May 24 about reports that South Africa was secretly selling arms to Argentina, and Mr. Nott promised to check.