"Executive I Foxtrot," the FAA code word for the jet that carries Nancy Reagan, landed here last night with a cargo including a first friend, a hairdresser, a nurse, a stack of hatboxes, racks of ball gowns -- and one secret outfit she plans to wear for the royal wedding next Wednesday.

"I mean," observed Nancy Reagan about her royal wedding attire, "you're not just picking a dress for any old thing, are you?"

The mood aboard the Boeing 707 was not unlike that of college girls going to debutante party. Nancy Reagan had a makeup case and four hatboxes with her in the plane's stateroom. Hairdresser Julius Bengtsson called it the "the ultimate in flying." And Betsy Bloomingdale, Nancy Reagan's friend, sat next to a shopping bag that contained her Marc Bohan black and white dress, carefully rolled in plastic.

"If you were to squash this down into a suitcase," she explained, "oh, I can't tell you. You'd never get the wrinkles out."

"I'm sick of that one," said her husband pleasantly. His name is Alfred. "Tell about the other one."

Bloomingdale looked at her husband, who was doing a crossword puzzle. "He's a character," she said. 4tJust through the closed doorway was Nancy Reagan. Here's what you could find in her stateroom: a big, baby blue, crushed-velvet chair. A small bed for napping. A private changing room. A desk, with notepaper. A picture of her and her husband swinging in a hammock. Fresh fruit. And jellybeans.

Nancy Reagan was traveling without her husband, who stayed in Washington to work. Both were invited to the wedding, but the White House felt the president's first trip overfseas should be more substantive. So Nancy Reagan came by herself, and her stay will be the longest she's ever spent away from her husband since they were married.

"I started to miss him last night," she said. "We've never done this before."

Further back on the plane was Julius, hairdresser to stars. Once he had to get up on an elephant with Dinah Shore to do her hair at Busch Gardens.In contrast, yesterday's conditions were a snap.

"I've done her hair in the bathroom on commercial flights," he said of Nancy Reagan, his Sony Walkman with a Donna Summer tape resting on his table, "and you know, on regular airplanes, the bathrooms are so small. This, of course, is much easier."

Julius fingered his "presidential guest" pin, worn on his maroon tie that went with his pink and white striped Ralph Lauren shirt. "I'm spoiled, you know," he said. "I hate to travel alone. I don't get treated as nice. It's not so fun."

About an hour and a half into the seven-hour flight, Nancy Reagan left her stateroom to talk to reporters. One of the first questions: Was she worried about wedding security?

"That's a silly questioin to ask me , isn't it?" She laughed.

But. . .

"I worry," she said, "and, I. . ." She paused, then sighed. "I worry."

Next somebody asked the standard: Was she excited?

"Oh, sure, of course, my gosh," she said. "When am I even ever going to see anything like this again?. . . I've never been to a polo match. I've never been to Buckingham Palace, well, the whole thing. I've never been to a royal wedding."

For the plane ride, Nancy Reagan wore a red Adolfo suit with a red and black dotted silk blouse. Somehow it never got wrinkled. For the wedding and her week of parties in London, she has brought . . . well . . . the White House staffers traveling with her wouldn't say what exactly she has brought, or how much. It appeared to be a delicate matter.

So staffers suggested that the best way to determine a ballpark estimate of outfits would be to count the number of events Nancy Reagan would be attending, then add a few "bet-hedger" dresses. Doing the arithmetic, this came out to around 20 dresses. Eight were figured to be ball gowns. There are no repeat outfits.

Nancy Reagan will be attending enough parties, dinners and luncheons for the next week to make even the strongest inaugural week veteran pale. It begins tonight, with a party at the estate of H. John Heinz II, of the Heinz (ketchup) Co. Then there's a dinner with Princess Alexandra, a polo match where she'll watch Prince Charles play, a dinner at the American Embassy, two more luncheons, a visit with the queen mother. And that's just through Sunday.

Nancy Reagan spent a fair portion of the past week packing and getting ready, as did most of her entourage. Everyone became consumed with last-minute details. The first lady's press secretary, Sheila Tate, for instance, ran out to Lord & Taylor in Washington Tuesday night to buy a sweater, only to be paged there by a White House advance person in London. On the night before, she left the office early so she could buy a blouse at Tyson's Corner. "I panicked," she said.

Nancy Reagan arrived at Heathrow Airport about 5 p.m. Washington time (10 p.m. London time) in a cold, drizzly rain. She walked down the plane steps under a clear umbrella held by a staff member, the lights of the plane shining on "United States of America" as it was written across the body of the Boeing. A group of local important people shook her hand, including American Ambassador John Louis. Then she was whisked away.

The day had begun at 10 a.m. Washington time. Nancy Reagan arrived at Andrews Air Force Base in a black limousine with Tate and Julius. Betsy Bloomingdale, who was already aboard, watched from the window.

"Oh, this is so exciting," said Bloomingdale. "I mean, really."

Bloomingdale also had a hatbox with her, which contained an orange and a beige straw variety. Upon request, she opened the box and let a few people have a peek.

"I hate hats," said her husband, unperturbed."But if you have to wear them to the wedding . . ."