Secretary of State Cyrus Vance was flat. Ann Landers missed her cue. Lynda Bird Robb sang in a high-pitched squeak. And everybody else joined in for the chorus of the very British "Twelve Days of Christmas," sung lustily after a festive state dinner at the White House last night.

It was all in honor of another vistor sent by the Mother Country to America, that once-rebellious little outpost across the Atlantic. This time, the vistor was a woman who reminded George Washington's successor that America, after all, was but a chip off the old Crown.

"I hope you don't mind, Mr. President," British Prime Minister Margaret Thatcher told some 140 guests collected in front of red poinsettias in the White House East Room, "if I recall that George Washington was a British subject until well after his 40th Birthday."

Thatcher, in town for about 40 hours, appeared buoyant after yesterday's news that the Patriotic Front in Rhodesia had agreed to a cease-fire. "I don't pretend that anyting is going to be remedied immediately," she said about Britain's longstanding problem, "but we are determined upon change."

Still, most of Thatcher's remarks before the dinner were touched with charm and humor. She even acknowledged the importance of a local addiction called the Redskins, and apologized graciously for prying Vance away from an afternoon in front of the tube.

"He had to take his eyes off the game to greet me," she said, informing the laughing crowd that her visit to Washington had thus "got off to a rather shaky start."

Jimmy Carter, despite Iran, appeared almost as buoyant. Introducing Thatcher, he quoted from Charles Dickens' "Pickwick Papers": "She knows what's what, she does."

And a little later, he pulled this from Walt Whitman: "I hear the running of the Thames river in my speech." The crowd snickered. So he added: "Perhaps you can't hear the running of the Thames in mine -- I'm from Georgia. But we have part of Britain in us."

After dinner, over coffee in the Green Room, the president admitted to reporters that he was a little discouraged by events in Iran.

"The news out of Iran is not good," Carter said. Asked if he thought any of the hostages would be released by Christmas, he answered, "I don't believe so.

"The students said no to everyting Ghotbzadeh said Sunday and Khomeini went along with them," the president said. He added that he was more discouraged than he had been over the weekend "because we understood that Ghotbzadeh met with Khomeini at Qom before he made those statements. We get mixed signals from Iran."

But Iran was forgotten, at least temporarily, during Christmas carols in the East Room. That's when 12 preselected and horribly self-conscious audience members trooped up under the grand chandelier to sing verses from "The Twelve Days of Christmas."

Vance, for instance, was in charge of "six-geese a-laying," while Kirk Douglas sang "seven swans a-swimming." Robb warbled "twelve lords a-leaping" and Landers tried valiantly with "eight maids a-milking."

"Oh, I lost my cue," she stammered at first, the piano coming to a grinding halt. But then, with renewed vigor, off she went.

The president stayed out of the limelight, which was just fine with him.

"I'm the world's worst singer," Carter said. "I can't carry a tune at all."

Douglas, on the other hand, was delighted with his own White House debut. "I've got it made," he decreed. "This is starting up a whole new career."

Douglas didn't know it, but while he was singing the groundwork for a new career, several audience members were deep in discussion about the cleft in his chin. Jerry Parker, a doctor from San Fransicso, told all those around him that he, for one, thinks Douglas has had the cleft adjusted. It was deeper in the movies, he determined.

While Thatcher's daughter Carol, 25, an Australian journalist, poked around virtually unnoticed ogling the decorations ("It looks like a fairyland," she said), the prime minister joined the Carters in front of the Christmas tree for a quick dialogue over coffee. She was excited to be there, she said, but also over the fact that the Rhodesian settlement had finally come.

"I don't ride, but people who do tell me the last fence is the hardest," she said.

Her urbane and elegant foreign secretary, Lord Carrington ("He's a honey," said Assistant Secretary of State George Vest) said the lifting of sanctions against Rhodesia by the United States over the weekend had "a great effect."

"It was realized that if the United States felt the settlement was fair it was going to be difficult to persuade other countries that it was not. Speaking for myself," Carrington continued, "it was a very brave decision by the president."

Carrington said there had been no quid pro quo -- "not a bit, not the smallest bit -- in the lifting of sanctions against Rhodesia by the U.S. in exchange for British assistance against Iran.

"We really have done quite a bit in the Iran situation -- we have a mission in Tehran," he said. "You can't say everything you've done, of course."

Besides Rhodesia, the day's other good news for Carter was passage of the windfall profits tax. "Congratulations, I heard it passed," he called out to Sen. John Glenn (D-Ohio), one of several senators who missed dinner because of the late vote.

As for politics, the president alluded to it by describing a cartoon that sits on Rosslynn Carter's desk. It shows two babies sucking their thumbs.

"One baby says, 'My momma says I can run for President.' And the other baby says, 'My momma is gonna rub out the ayatollah.'" The president laughed.

Thatcher arrived at the White House at 7:20 p.m., driven across the street from Blair House in a big fat black limousine. Nearby, in Lafayette Square a giant menorah had just been lit by the president, commemorating Hanukah. As Thatcher got out of her limousine, a small crowd waving Irish flags near the menorah chanted, "Brits out, Brits out." Thatcher paid them no attention as she said "Good evening" to Jimmy Carter.

Inside, the White House Grand Foyer sparkled with Christmas decorations. Two small trees with red ribbons and hanging apples flanked the entranceway, green boughs traversed the walls and the Marine Band played "Rule Britannia."

To the left was the East Room, where guests were announced by loudspeaker. Secretary of Defense Harold Brown chatted with presidential counsel Lloyd Cutler and American Motion Picture Association president Jack Valenti gave a hearty shake to Kirk Douglas' hand.

Other guests included Vice President Walter Mondale and wife Joan; Secretary of State Vance and wife Grace; several senators and representatives, and some present and past U.S. ambassadors to the Court of St. James's, including W. Averell Harriman and his wife Pamela.

Her son, Winston Churchill III, figured prominently yesterday in London headlines that linked him with the ex-wife of billionaire Saudi Arabian arms dealer Adnan Khashoggi. Churchill is a member of Thatcher's Conservative Party and serves in Parliament where he was junior defense spokesman at the time of his admitted affair with Soraya Khashoggi.

When Pamela Harriman arrived to the tune of the White House harp last night, she was asked if she was proud of the courage her son exhibited in disclosing his identity in the affiar. She responding by smiling broadly and saying: "Sure, I agree with his statement."

The evening was purposefully English. Nosegays reposed at each dinner plate (just like they did at the White House dinner for Thatcher's predecessor, James Callaghan) and Christmas carols were sung.

If the music was British, the food was French. Guests ate pheasant veronique, wild rice with mushrooms, brussels sprouts and yule logs with sabayon sauce. Guests at Last Night's Dinner

The following were guests at last night's state dinner for Margaret Thatcher: Miss Carole Thatcher Lord Carrington, KCMG, MC, Secretary of State for Foreign and Commonwealth Affairs Sir Robert Armstrong, KCB, CVO, Secretary of the Cabinet Sir Michael Palliser, GCMG, Permanent Under Secretary of State, Foreign and Commonwealth Office Sir Frank Cooper, KCB, CMG, Permanent Under Secretary of State, Ministry of Defense The British Ambassador & Lady Henderson Mr. Michael Pattison, Private Secretary to the Prime Minister Mr. Bernard Ingham, Press Secretary, No. 10 Downing Street Mr. Michael Alexander, Private Secretary to the Prime Minister Mr. George Walden, Private Secretary to the Secretary of State Air Marshall Sir Roy Austen-Smith & Lady Austen-Smith, British Defense Attache Mr. & Mrs. Derek Thomas, Commercial Minister, British Embassy Mr. & Mrs. Martin Morland, Counselor, British Embassy Vice President & Mrs. Mondale Secretary of State & Mrs. Vance Secretary of Defense & Mrs. Brown Hon. & Mrs. Zbigniew Brzezinski. Assistant to the President for National Security Affairs Sen. & Mrs. Frank Church (Idaho) Sen. & Mrs. Thomas F. Eagleton (Mo.) Sen. Joseph R. Biden Jr. (Del.) Sen. & Mrs. John H. Glenn (Ohio) Sen. & Mrs. Malcolm Wallop (Wyo.) Rep. Clement J. Zablocki (Wis.) Guest: Ms. Dolores Ecklund Rep. & Mrs. Jack Brooks (Tex.) Rep. & Mrs. John P. Murtha (Pa.) Rep. & Mrs. Harold E. Ford (Tenn.) Hon. Sarah Weddington, Assistant to the President Guest: Dr. Jacob Joseph Parker Hon. & Mrs. Lloyd N. Cutler, Counsel to the President Admiral & Mrs. Thomas B. Hayward, USN, Chief of Naval Operations Hon. & Mrs. Kingman Brewster, American Ambassador to Great Britain Chief of Protocol & Mrs. Valdez Hon. & Mrs. George S. Vest, Assistant Secretary of State for European Affairs Mr. & Mrs. Robert D. Blackwill, National Security Council Staff Member Hon. & Mrs. Averell Harriman, Former Ambassador, Washington, DC Hon. & Mrs. Walter Annenberg, Former Ambassador, Wynnewood, Pa. Mrs. Davd K. E. Bruce Mr. Arthur D. Avila, Van Nuys, Calif. Guest: Jonathan David Avila, son Mr. & Mrs. William Bowers, San Antonio, Tex. Mr. John Brown, Calhoun, Ga. Mr. & Mrs. Thomas J. Brown Sr., Calhoun, Ga. Ms. Sarah Caldwell (Guest: Mr. Donald Gramm) Conductor, Boston, Mass. Mrs. John W. Carter, Calhoun, Ga. Hon. Martha Layne Collins & Dr. Bill Collins, Lieutenant Governor of Kentucky Mr. & Mrs. John Cowles, Jr., President Minneapolis Star and Tribune Co. Mr. & Mrs. Robert Dickey, III, Chairman, Dravo Corporation, Pittsburgh Mr. & Mrs. Kirk Douglas, Beverly Hills, Calif. Mr. & Mrs. Ronald D. Eastman, Attorney -- Cadwalader, Wickersham & Taft, Washington, DC Mr. & Mrs. William C. France, Daytona Beach, Fla. Mr. & Mrs. J. B. Fuqua, Chairman, Fugua Industries, Inc., Atlanta Mr. & Mrs. Edward J. Goett, Pres., ICI North America Limited, Wilmington Hon. & Mrs. Gilber Hahn Jr., Washington, DC Dr. Maria Hernandez, Miami, Fla. Guest: Ms. Yvonne Santa-Maria Mr. James Hoge, Editor-in-Chief, Chicago Sun-Times. Guest: Ms. Elizabeth Drew Mr. John Holloman, Jackson, Miss. Guest: Ms. Edith Ann Holloman, daughter Mr. Cordell W. Hull, Vice Pres. & Treasurer, Bechtel Corp., San Francisco Mr. & Mrs. John H. Johnson, Pres. & Pub., Johnson Publishing Co., Inc., Chicago Mr. & Mrs. Bill Kovach, New York Times Hon. Madeleine Kunin, Lieutenant Governor of Vermont & Dr. Arthur Kunin Mrs. O. C. Langford, Calhoun, Ga. Mrs. Eppie Lederer, Columnist Ann Landers, Chicago. Guest: Dr. Robert Stolar Hon. & Mrs. Joe C. McCorquodale, Jr., Speaker of the Alabama House of Representatives Mr & Mrs. Willard H. McGuire, Pres., National Education Association Hon. C. J. McLin, Jr., Ohio State Representative. Guest: Ms. Virginia Dickens Mr. & Mrs. Sylvan Meyer, Publisher, Miami Magazine Mr. Gordon W. Mills, Los Angeles, Calif. Guest: Ms. Anna-marie Brekke Mr. & Mrs. James T. Mills, President, Sperry & Hutchinson Company Hon. Carl Officer, Mayor of East St. Louis, Ill. Guest: Mrs. Myrtle Officer, mother Hon. & Mrs. Charles S. Robb, Lieutenant Governor of Virginia Mr. David M. Roderick, President, United States Steel Corp., Pittsburgh Mr. & Mrs. Pepper Rodgers, Coach, Georgia Tech Football Team Mr. & Mrs. Harold Roitenberg, Minn. Mr. & Mrs. Steven J. Ross, Chairman, Warner Communications, New York Ms. Madeleine Haas Russell, San Francisco, Calif. Guest: Charles Russell, son Mr. & Mrs. Howard J. Samuels, Howard Samuels Enterprises, New York Hon. & Mrs. Robert S. Strauss, Chairman Carter/Mondale Presidential Committee Ms. Louise M. Sunshine, New York, New York. Guest: Mr. Lee Elman Hon. & Mrs. Jack J. Valenti, President, Motion Picture Association of America, Inc. Mr. Warren Weaver, Jr., New York Times & Ms. Marianne Means, King Features Syndicate Mr. & Mrs. Jack Welch, Dallas, Tex. Hon. & Mrs. John C. White, Chairman, Democratic National Committee