London — Following the British landing on the island of South Georgia, that mountainous chunk of rock suddenly appears more strategic than many had previously realized.
Ninety miles long and 25 miles wide, it offers British strategists spacious harbors free from ice and sheltered from the rolling swells of the South Atlantic by mountains that rise to a height of 9,600 feet.
On Sunday it became the focus of attention as British troops went ashore there and as Britain reported that two helicopters from its destroyer flotilla near the island had attacked and damaged an Argentine submarine. The submarine had appeared within the blockade zone around the naval task force.
South Georgia is outside the range of Argentine aircraft based on the mainland more than 1,000 miles to the west.
Its main harbor, Grytviken, contains huge storage tanks that once held whale oil in the days when whaling fleets used the island as a major base. These tanks could now be used to hold fuel for the British task force.
British ships could even be repaired there if necessary. John Matthew, former British administrator of the island, says that the propeller on a large Russian whaling ship was changed there in 1958.
Mr. Matthew also sees considerable, but often overlooked, strategic advantages in holding South Georgia.
The only harbors in the South Atlantic to compare with it, he says, are those of the Magellan Strait -- and Simonstown in South Africa. In radio interviews here and in a letter to The Times of London he added:
''It only needs one bomb gn Suez and another on Panama for it to be demonstrated that whoever has South Georgia can dominate the South Atlantic and western Antarctica. Until now, peaceful British administration of South Georgia has made this risk quite obscure.''
The European Community, he says, ought to be more aware of this situation.
''If two frigates can be made available for the South Atlantic, and one kept there continuously, South Georgia can continue to be a kicking-off point for further peaceful exploration in Western Antarctica and will be denied to a possible enemy in times of war. This is a valuable contribution for Britain to make to the European and North American communities.''
One other advantage of regaining South Georgia is suggested by Lord Shackleton, grandson of the famous explorer, Sir Ernest Shackleton, and now a Labour peer in the House of Lords. He says that the area is the center of the richest grounds in the world for krill (small shrimp-like crustaceans) and contains amounts equal to the world's entire fishing catch.
A landing on South Georgia would allow Margaret Thatcher to raise the British flag again, take Royal Marines from the heaving decks of the task force and put them on dry land, and raise pressure on Buenos Aires.
British sources say Argentina may only have 50 men on South Georgia. Other sources put the number at 100 or more. It was assumed here, however, that a landing there might succeed without risking large loss of life.
(According to Reuter reports from London, authoritative sources said about a dozen men of the navy's crack Special Boat Squadron were landed on South Georgia on Thursday from a British submarine.
(The sources said the commandos prepared the way for the main force to land Sunday. They added that the main force numbered 44 men, plus a mortar team. They said the advance team radioed details of Argentine positions back to the task force.)