The British are coming -- and this time Washington rolls out the red carpet. Pomp and circumspection surround the royal couple

FROM the moment the big silver VC-10 touches down at Andrews Air Force Base at 8:45 tomorrow morning, the Prince and Princess of Wales Diana will be off on a courtly marathon. Like paratroopers or foreign correspondents, they will hit the ground running, zooming off through the red and gold foliage of the Maryland countryside to the brick residence of the British ambassador in Washington, Sir Oliver Wright, and Lady Wright. With barely time to drop off the royal luggage, they will then motor to the White House for a private meeting over coffee with the Reagans.

That's the beginning of a nonstop four-day tour that includes 17 official functions, from the White House gala dinner that night -- with Robert Redford, Diana Ross, Clint Eastwood, and Peter Ustinov reportedly among the guests -- to a J. C. Penney ``Best of Britain'' promotion of British goods at a Springfield, Va., shopping mall. A major stop on the marathon will be the gilded ``Treasure Houses of Britain'' exhibit at Washington's National Gallery, for which the royal couple are patrons.

But it will not be all pomp and circumstance: Princess Diana will join Nancy Reagan Monday on a trip to Straight Drug Rehabilitation Center in Springfield and join Barbara Bush, the vice-president's wife, in a tour of the Washington Home and Hospice.

Prince Charles will visit the American Institute of Architects Saturday for a tour and private talks on community architecture with groups from Baltimore and from Savannah, Ga. On Sunday he will ``read the lesson'' (from Isaiah 35) in an early-morning service at the Washington Cathedral. Afterward, the couple will drive out to Virginia hunt country for an Upperville luncheon with philanthropist and art collector Paul Mellon.

The most highly guarded state secret here this week is what's on the menu for that luncheon and for the magnificent dinners at the White House, the British Embassy, and the National Gallery.

At the embassy, a spokesman says with diplomatic discretion that no word could be leaked on Sunday evening's dinner for the couple and the Reagans -- or on whether there are certain foods that do not please the royal palates and will not be served anywhere during their US visit.

``Recently the Prince indicated he prefers fish, and she [Princess Diana] does, too, very often, to meat,'' British Embassy spokesman John Hughes says. ``But he's not a thoroughgoing vegetarian -- he does eat meat, but not a lot of it.''

Will they be breaking out the scrod or lobster at the White House? Mrs. Reagan's press secretary, Elaine Crispin, says, ``The White House normally never has fish as a main course at dinner, and I don't know whether there's any fish on the menu or not.'' For tomorrow evening's White House dinner for 80, which has been described as ``small and cozy,'' all questions about the menu, d'ecor, flowers, guest list, and entertainment are met with one inevitable answer: ``I'm not confirming or denyin g anything. All that information will be given out on Saturday.''

Throughout Washington there is a tight lid on all information about the royal visit, from trivia on up. And although the visit will bring on a regal stampede of ``herd journalism,'' most of the more than 500 reporters from the United States and around the world eager for a glimpse or a word with the Prince and Princess of Wales will have to write from pool reports.

All requests for interviews, including one from ABC's Barbara Walters (who previously interviewed the Prince), have been turned down. The Prince and Princess will meet a few members of the press Saturday afternoon at a British Embassy ``media reception.'' But reporters are asked to check their cameras, notebooks, and egos at the door; it's all off the record.

Most of the reporting pools will be small, five to 10 reporters, like the one at the National Gallery late Sunday morning, when the royal couple will visit the ``Treasure Houses of Britain'' tour by the gallery's director, J. Carter Brown. Small clumps of reporters will be stationed at a few vantage points, like the Van Dyck portrait of the royal Stuart brothers in the Jacobean Long Gallery and in the Waterloo Gallery. There will surely be a stop at the painting that Prince Charles and his mother, Queen

Elizabeth II, lent to the exhibit: John Wootten's 1740 oil, ``The Shooting Party,'' in which Frederick, Prince of Wales, on horseback, surveys a felled stag.

Some Washingtonians have been buffing their tiaras, anticipating coveted invitations to the rather grand dinner to be given Monday evening by National Gallery trustees. The Ford Motor Company is picking up the tab for the Treasure Houses exhibit (as much as $6 million).

Millions are also involved in the ``Best of Britain'' promotion, which embassy spokesman Hughes says involves $50 million worth of British clothing.

``It's quite normal for members of British royalty to support the country's trade,'' he explains, quelling questions about public criticism that J. C. Penney, which has 5-and-10-cent-store roots, is not exactly Harrods.

Out at the Springfield Mall, Penney's is gussying up the store for the visit: slews of red roses, a red carpet, British bunting and banners, and security guards in quasi-British uniforms (although they reportedly balked at dressing in Beefeaters' uniforms with red tights). Deborah Masten, spokeswoman for the store, says 500 stores in Penney's 1,700-store chain will be involved in the promotion.

At the Straight drug center, there will be no red carpet, no tea -- just straight talk from two of the home's graduates and their families: Kathy Turner and Mike Kirsch, who have kicked their dependence on using marijuana, alcohol, and drugs.

According to Linda Hedden, a spokeswoman for Straight, the focus of the visit will be a 40-minute open meeting -- a therapeutic session in which children with drug problems and their families ``renew family ties.'' Princess Diana, concerned with the rapid rise in drug use among British youth, had requested such a visit to a rehabilitation center.

The Prince, too, will be talking with a group involved with the social problems he is concerned about in the wake of recent urban rioting in England. He will have an off-the-record session with the American Institute of Architects' Regional Urban Design Assistance Team on how to turn problem neighborhoods into good places for people to work and live.

On Tuesday morning the pair are off to Palm Beach, Fla., for a polo match and a reception and dinner for the United World Colleges, before flying back to London.

Is there anything else, any morsel of news or any tidbit of trivia, that the British Embassy can throw to the press? ``I'm milked of trivia,'' sighs Hughes.