In the annals of British military history, the second battle of Grytviken is unlikely to loom large.

Captured Argentine military commanders "were entertained to dinner on board one of her majesty's ships" just hours after British Marines and commandos retook the port of Grytviken on South Georgia in the South Atlantic yesterday, a British Defense Ministry spokesman told an expectant press conference here today.

He did not describe the meal or the uniform of the day, but it has become obvious that the level of hostilities so far has been something less than intense.

Yesterday's battle was the third armed encounter between British and Argentine troops since Argentina began its invasion of the Falkland Islands and South Georgia more than three weeks ago. So far, just one British Marine has been hurt, suffering a slight arm injury, and fewer than 20 Argentines are believed to have been killed.

This pattern could change very soon, however. There are persistent rumors here that specially trained Marines already have landed secretly on the Falklands to reconnoiter. The press here has reported that a similar group carried out just that type of operation on South Georgia Thursday, three days before the island was recaptured.

Retired air marshal Paddy Menaul, a prominent military analyst, said in an interview he would not be surprised if British troops were already on the islands. If there were no progress by mid-week in negotiating an Argentine withdrawal from the Falklands, "we will have to go in and throw them out," he predicted.

His on-the-record remarks reflected what is being heard in political circles around London.

Prime Minister Margaret Thatcher's increasingly tough stance supported the expectations. In a television interview tonight she said, "There was a feeling growing up that we would not use" the large British task force dispatched to the South Atlantic three weeks ago. "It was obvious that we needed to recapture South Georgia."

The main part of the task force is now in the vicinity of the Falkland Islands, 800 miles west of South Georgia. It appears that only a few ships were used to retake South Georgia.

So far, Britain is emphasizing there is no state of war. Thatcher corrected a member of Parliament who asked her about the 156 prisoners of war. They are simply prisoners, she said.

Lt. Col. Tim Donkin, the tight-lipped Defense Ministry spokesman who spoke in staccato bursts but provided little information, said that the captured Argentine commanders expressed "great gratitude to the British captain for the humanity being shown to the prisoners."

The only casualty in the two-hour battle was an Argentine sailor seriously wounded in the leg when British helicopters strafed his submarine as it entered the remote, sparsely inhabited port of Grytviken.

"No Royal Marines like to kill people unnecessarily," Donkin said. "This operation was very carefully planned beforehand with the direct object of ensuring a minimum number of casualties."

To an incredulous questioner who asked if the troops aimed not to hit the enemy or if a "magical factor" was involved, Donkin said, "If we can achieve success by not injuring or killing any person that is greatly to all our advantage . . . and that is what happened."

Donkin said the hostilities began at dawn yesterday when British helicopters from warships off the coast were fired upon after deploying reconnaissance forces on the island. The helicopters spotted a surfaced Argentine submarine--the U.S.-built, World War II vintage Santa Fe--and called in attacks from other copters.

"Three direct hits were scored" on the submarine, which was disabled and beached. Then the ships at sea bombarded the island, additional Marines and soldiers were landed and "quickly established some superiority."

"The opposition appeared limited if not unenthusiastic," Donkin said and speculated that perhaps the fate of the submarine had dampened the Argentines' desire to fight.

The British troops "moved in resolutely" and within about an hour a white flag was hoisted, signifying the surrender of Grytviken.

Sixteen Argentine special force troops at the port of Leith, about 15 miles away refused to surrender so other British forces landed there by warship at dawn today. After a brief exchange of fire, the 16 surrendered, the spokesman said.

Thirty-eight civilians were also captured--mainly scrap merchants who came onto the island in mid-March without permission in an incident that precipitated the Falklands crisis. The British government said they will be returned to Argentina.

Donkin said the Argentines had planted land mines and booby traps. He would not provide any information about the British ships or helicopters used, nor would he tell the number of troops employed except to say "there were far less of us than there were of them."

The Argentine military forces now being held on the island consist of 60 crewmen from the submarine, 56 troops that had been garrisoned on the island and 40 reinforcements being brought by the Santa Fe.

Donkin refused to say whether the troops that attacked yesterday were the first to land on the island, thus lending credence to the widespread reports that 12 Special Boat Service Marines landed on the island Thursday.

Another Defense Ministry spokesman simply said, "We never talk about" the special forces. "We like to keep them under wraps. They are very good. Very, very good!"