British Defense Secretary John Nott strode beaming to the rostrum for his press conference this afternoon and immediately displayed his prize possession--a picture of a Royal Marine sipping a tin mug of tea and talking to a Falkland Island mother and her towheaded children.

"I hope you give this picture wide circulation around the world," he told reporters eager for news about the fate of the British invasion of the Falkland Islands yesterday. The picture made his point--that British troops were liberating women and children from their Argentine conquerors.

Then--taking advantage of a televised news conference on a day when Britons were glued to the television awaiting the year's biggest sporting event, the Football Association Cup soccer final--Nott said, "the major amphibious landing yesterday was a complete success."

After weeks of frustrating negotiations and rumors of war, the invasion was finally under way, and Nott was the personification of confidence. "We are back in the Falkland Islands and back in strength," he said. "We intend to ensure that aggression does not pay."

Asked what happens next, he added, "We're not just going to sit on our hunkers. We are going to repossess the islands."

In the mood of the moment, Nott saved the bad news for last. One frigate had been sunk, and about 20 men were missing, adding to earlier figures of three dead and 27 wounded in the day's fighting.

Still, after a night of tension following reports of heavy losses as a result of what were described as virtual "suicide raids" by Argentine jet pilots, it could have been much worse.

Britons breathed a noticeable sigh of relief. Michael Nicholson, a British television reporter with the Royal Navy task force, described the mood in the fleet that undoubtedly mirrored the attitude of many Britons. There is, Nicholson said, "unconcealed pride that so much was accomplished so quickly with so little loss of life."

Prime Minister Margaret Thatcher also reflected this attitude, saying, "Our boys have been absolutely magnificent, as you might expect."

She met with her "war cabinet" in a session dominated by consideration of the military situation, then went to naval headquarters at Northwood for a briefing before going to Chequers, the prime minister's country estate.

An eyewitness report of the British landing was provided by Independent Radio News correspondent Kim Sabido, who went ashore with the commandos from the troopship Canberra. In his report, which was subject to military censorship, Sabido said:

"As the sound of air and ground attacks still echoed around the bay, we went ashore by landing craft, wading through the water for the last few feet to set foot for the first time on Falklands soil. We marched for three kilometers about two miles across boggy, windswept terrain and fanned out with constant helicopter support.

"As the weather changed and rain came in we reached a settlement. Some 50 locals, including up to a dozen children, were busy ferrying ammunition to gunners using their tractors and trailers, dishing out soup and tea to the troops and providing much-needed shelter.

"All the time, Argentine jet fighters screamed overhead. Gray and black smoke billowed from around the anchorage."

The British popular press was quick to exult, in some cases even before the casualty toll was known.

The Sun, the country's largest circulation daily, ran a "2 a.m. special" to publish a front-page picture of Marines hoisting the Union Jack at San Carlos. "The flag of freedom fluttered again over the Falkland Islands," the newspaper said, calling the landing "probably this country's most historic moment since the German surrender of 1945."

So momentous was the event that The Sun altered its traditional war makeup. The headline on pages two and three calling it "The paper that supports our boys" gave way to "Back where we belong." The paper ran a headline "Just for you, Galtieri," a reference to Argentine President Leopoldo Galtieri, accompanied by a file photo of a Scorpion light tank in action.

The paper's daily topless beauty, who normally unfurls herself to readers on page 3, was pushed all the way back to page 13.

The Daily Express headlined its story "We're back." It also focused on problems for the monarchy in the crisis, particularly the fact that Prince Andrew, second son of Queen Elizabeth II, is a Sea King helicopter copilot in the task force.

Seven of the Sea Kings have crashed due to mechanical problems, causing the newspaper to say that the queen is "now caught in the most worrying and poignant situation of any monarch this century."

The paper also was concerned that Prince Andrew's sister, Princess Anne, may have had to shake hands with Argentine soccer star Ricardo Villa at today's cup final. The princess was introduced to both teams, but Villa, who has been subjected to booing and catcalls since the confrontation with Argentina, decided not to play for his team, Tottenham Hotspurs.

An official of Wembley Stadium, where the match was played, said, "There is no way he could have played when there is so much fighting going on around the Falklands. It would have been highly embarrassing for Princess Anne to have had to meet him."

To some extent, the match resembled the weeks of agonizing waiting for war between Argentina and Britain until yesterday. Tottenham and Queens Park Rangers drew 1-1 in overtime, and the whole match will have to be replayed Thursday, giving the controversy about Villa's participation more time to simmer.

Falkland Islanders in Britain were quick to welcome the return of the British flag. A statement issued by the Falkland Islands Office in London called on Argentina to withdraw its forces now to avoid more loss of life.

Concern over possible casualties in yesterday's fighting did not, on the surface, dampen weekend spirits. Aside from the sellout audience at Wembley, crowds thronged to London's parks and large numbers watched the changing of the guard at Buckingham Palace. In contrast to street scenes of Buenos Aires shown on television, nobody here seemed to have a transistor radio pinned to his ear to get the latest news.

Perhaps the waving of the pictures by Defense Secretary Nott was a key to the British sense of confidence over the invasion. For weeks, the press and the Defense Ministry had been involved in a miniwar over the Royal Navy's refusal to allow pictures to be transmitted from the task force.

The ministry had cited operational and technical difficulties, but last night, with a victory in hand, the pictures were transmitted in time to make the morning editions.

Another sign of success was that American correspondents were allowed into the ministry for a rare unattributable briefing.

Many of the office doors were adorned with stickers saying, "Save our Falklands."

There was also evidence that the British have been at this military business for a long time. In one hallway of the ministry, on the site of an old royal palace, a sign gives directions to reach the wine cellar of King Henry VIII.