The American tourist was describing the private little dinner party he had just come from to some of his fellow countrymen here Saturday night.

"I think the whole royal family was there," he said. "I had a good place to sit. I was between the queen and Princess Margaret, and across the table was Prince Charles and Prince Philip and the queen mother was there too . . ."

The tourist was Jimmy Carter, former peanut farmer from Plains, Ga., who recalled that the only other time he had been in London he had to peer at Buckingham Palace "through the fence." But Saturday night there let him in through the front door, and if there was a slightly giddy, awe-struck tone to his recollections of the evening, there was good reason for it.

The President is being treated royally here. In a city where there are now seven heads of government and a press corps of more than 1,500 to report their summit meetings, he is the constant center of attention, the one the crowds turn out to see and to wave at.

"He's a knock out. Why compete?" laughed Prime Minister Jame Callaghan as the President waded into the crowds, campaign-style, outside the prime minister's residence at 10 Downing Street.

He arrived here Thursday night, the newly-elected American President, still something of a stranger to his fellow "summitteers," as the seven are called here, and to the people of their countries. Through two days of weighty discussions on economic and related issues, however, Carter has left the unmistakable omprint of his style on this capital. From all accounts, he has won a huge amount of goodwill for himself in the process.

This morning he was up early to attent services in the grandeur of West-minster Abbey, where he had to borrow money from an aide for the collection.

After the 40-minute service, the President had a tour of the building, taking the opportunity to press his guide, Bishop Knapp Fisher, for some official recognition of one of his favorite writers, the Welsh poet Dylan Thomas.

In the Poet's Corner of the Abbey, where some of England's greatest writers are entombed, Carter said that he thought Thomas should be similarly honored. Perhaps, the bishop replied, in time Thomas, who died in 1952 and was known for his public carousing, would make it to the abbey "whatever his morals."

"I would like to recommend it," the President said. "I will pray for his soul if you will commemorate him."

Saturday afternoon, Carter did another characteristic thing. He walked to luch, leading the other summitteers from No. 10 Downing Street, site of their meetings, across St. James's park to Lancaster House. The expected chaos when a President takes a stroll in public ensured.

The British press has been enthralled by all of this. Reports of Carter's tour of northeast England Friday with Callaghan included headlines such as "Mr. Carter Wins Hearts of the Northeast" and "100,000 Georfies Greet Carter the Charmer."

White HOuse press secretary Jody Powell said that Carter was in an exciptionally good mood as he passed the midway point in his first trip overseas since taking office.

"We've taken care of a number of things," Powell said. Noting that a much-publicized possible personal feud between Carter and West German Chancellor Helmut Schmidt never materialized when the two met here Saturday. The meeting went so well, Powell said, that afterward the "We can pack our bags and go home while we're head."

Tonight, following the end of the two-day summit, Carter returned to Winfield House, the opulent residence of the American ambassador when he is staying Smiling broadly, he told reporters he is "quite satsified with the results of the summit.

On Monday, Carter will turn his attention to Berlin and he Middle East. He will meet first with Schmidt, Callaghan and French President Valery Giscard d'Estaing to discuss the situation in West Berlin.

Asked about the meeting tonight, the President agreed with a questioner that the Berlin Wall "makes a mockery" of the four-power post-World War II agreement granting freedom of access throughout the city.

"I think the wall is a very dramatic indication of the hunger for freedom among people who live in Eastern Germany," he said.

Monday afternoon, Carter will fly to Geneva for a meeting with President Hafez Assad of Syria to discuss the Middle East.

(In Geneva, Assad said he hoped his meeting with Carter results in a "new push toward peace" But aides said Assad had a "lot of questions," on Carter's new Middle East policy, United Press International reported.

(Assad arrived in Geneva Sunday following a late night meeting with Palestinian leader Yasser Arafat Saturday. Carter and Assad will meet for four hours Monday.)