Why doesn't Nancy Reagan seem tired? Today she:

Had lunch with Princess Margaret at Kensington Palace.

Saw the Dance Theatre of Harlem perform at Covent Garden.

Returned to Winfield House for 45 minutes to change from one long gown to another.

Then went to the queen's ball at Buckingham Palace, the most stellar social event of this madding week.

She has 36 hours, two dinner parties, two luncheons, one reception and one tea to go. That doesn't include the royal wedding itself. The royal couple will leave soon after for their Mediterranean cruise but everyone else -- including the first lady -- will hang around for the after-parties.

Tonight, the queen's ball (although the invitation termed it a "reception") was the talk of the upper class. "It is the ball," said Tina Brown, editor of the glossy gossip magazine The Tatler, "all the top drawer."

It looked like something from an 18th-century English romance. Buckingham Palace was awash with light; banners were draped from poles around the grounds. The nobility arrived in a slow-moving line of cars, as if in formal procession. Literally thousands of people watched through the wrought iron, clinging to the bars as the guests passed through the giant gates.

Crowd estimates for the ball were put at 1,500, but Buckingham Palace, cagey as ever, would neither confirm nor deny. Most of the royalty and a large slice of the European aristocracy were on the guest list, as well as friends of the queen, Prince Philip, Lady Diana and Prince Charles. Most of his ex-girlfriends were on the list, too -- including the jet set's own Sabrina Guinness.

The ball -- where the popular group Kenny Ball and his Jazzmen played -- was expected to last until 3 or 4 a.m. Tiaras were rampant.

"I have mine sitting on my desk," Lady Elizabeth Shakerly said today, referring to her headpiece. Lady Elizabeth is a key player here this week. Not only was she to attend the queen's ball, but she also will be giving Wednesday night's after-party at Claridge's.

"We haven't worn tiaras to a royal ball for a long time," said Lady Elizabeth. "They've cut them out since, heavens! Princess Anne's wedding, wasn't it?"

But now they're back. "I think everybody who had one," said Lady Elizabeth, "got rather sad that it was sitting in the bank all the time."

The tiaras underscored one point: The ball was for the aristocracy, not heads of state. But Nancy Reagan was invited. How this happened was a matter of some puzzlement because French President Francois Mitterrand, for instance, was not.

Nancy Reagan arrived sans tiara, but in long white evening gloves and a white-beaded, off-the-shoulder Galanos gown. Her hair was done by Monsieur Marc, apparenmtly with Julius' knowledge and consent. "The hair," as Julius refers to it, was swept high off her forehead in a dramatic pouf.

She also appeared to be wearing elaborate diamonds earrings. "Are they real?" an nosy reporter yelled out as the first lady stepped out of Winfield House.

"I'll never tell," Nancy Reagan replied.

Her good friend Betsy Bloomingdale was also on the guest list. Several years ago, she had the uncanny good fortune to meet Lady Diana's parents. So she was there as a friend of the family.

Before the ball there was a much smaller palace dinner given for 80 of the closest royal relatives and friends. Charles' friend Nicholas Soames was expected there, as were some of Lady Diana's close friends. Nancy Reagan arrived afterward, about 10:30 p.m., after watching the Harlem ballet perform.

When she arrived at the Royal Opera House, earlier, wearing a red Jean-Louis gown, she was cheered, and booed, by the crowd. The cheers came first, then the boos, which seemed to swell from one side of the street. Nancy Reagan, whose reception from the British press has been less than warm, had no reaction. It wasn't clear that she'd even heard it.

Inside the Opera House at intermission, Nancy Reagan went backstage to speak with the dancers who performed at the White House earlier this year. "It's so nice to see you all again," she said. "We've started to be a team. And I'm sorry I can't stay for the whole ballet, but I have to go to the palace. You understand."

Earlier today, Mrs. Reagan laid a wreath in honor of American and British servicemen at the American memorial chapel inside St. Paul's Cathedral. The place is cavernous. Today it was full of workers preparing yards of flowers and plants. Television cameras and monitors poked from behind marble columns. There are 8 1/2 miles of electric cable just inside the cathedral.

About noon, a crowd of 2,000 had formed in front of the cathedral steps, flowing onto the sidewalks of Ludgate Hill. People were doing that everywhere, hoping to see the hoopla without the crush of wedding day.

Nancy Reagan's motorcade drove up at 12:15. "Six cars," said a startled Frank Harvey, a canon of the cathedral. Nancy emerged, meticulously dressed in a black and white Galanos dress with a giant ruffled flower, a black straw hat and black patent shoes, accented with white patent bows across the tops.

She waved to the crowd and the crowd, in the customary British fashion, waved back silently. No cheers. No whistles. It's not considered polite.

After the wreath-laying, she had a quick tour. One stop was the vestry, where the Very Rev. Alan Webster asked her to sign the register.

She sat down at the desk, neatly wrote out "Nancy Reagan," then stared at it with wide eyes.

"Or would you rather have had me sign 'Mrs. Ronald Reagan'?" she asked Webster, eyes still wide.

He told her no, that was fine.

She looked up at him again, eyes even wider.

"Is that all right?" she said.

He said, yes, it was.

She looked toward the collected church staff, her own staff, and reporters. "I feel self-conscious," she said.

Back out on the steps, she was questioned by more reporters.

"Mrs. Reagan, what dress are you going to wear to the wedding?" several called out.

"It's going to be released tomorrow," she replied.

"Are you looking forward to the wedding?" someone else called out.

"I certainly am," she replied. "Isn't everyone?"

Then she drove off.

Other developments:

British reporters seem to have found American reporters here this week a bit aggressive.

On the front page of today's Daily Telegraph, for instance, a story about the polo match Prince Charles played in on Sunday said American reporters covering the first lady were wrestling to try to get through the crowd and have the best view possible.

"British security men," wrote the Telegraph, "who had the temerity to try to stop the American press corps, had 'the trip of the first lady' badges waved in their faces. 'This,' shouted an outraged American woman, 'was issued by the White House.'"

A pigeon, named Basil by one of the tabloids, turned up in St. Paul's and was caught over the weekend. The Royal Society for the Prevention of Cruelty to Animals got him with a butterfly net.

Today, while waiting for Nancy Reagan, some of the canons were questioned about Basil. How, for instance, did a pigeon get into the carefully guarded cathedral?

"How?" said Alf Parker, the canon's verger. "Walked in, I suspect."