First Lady Nancy Reagan made a slight slip in royal etiquette yesterday.

Mrs. Reagan, fresh from a cheering reception from hundreds of wellwishers outside St. Paul's, told an American television reporter how much she was enjoying her week-long visit to Britain.

She said had enjoyed meeting all the royal family -- including "the king and queen."

But Mrs. Reagan immediately corrected the slight mistake -- "I mean, Prince Philip and the queen."

Prince Philip, Queen Elizabeth II's husband, is not the king of England. The queen succeeded to the throne in 1952 on the death of her father, King George VI, and royal etiquette dictates her husband can only take the title of prince consort, and not that of king.

Lady Diana's wedding dress is being guarded round the clock in fear of a commando-style raid to steal it, the designers said yesterday. Cindy Watson, a spokesperson for the Emanuel brother-and-sister design team, said they had been offered up to $70,000 by British newspapers for the design.

She said the designers had heard rumors of a commando raid on the workshop to get the dress. "Obviously we're worried about it," she said. Work on the dress is complete except for a few final stitches to be taken tomorrow -- just before the ceremony, for luck.

Kingman Brewster, a former U.S. ambassador to Great Britain under Jimmy Carter, said yesterday in New York that the royal wedding could be "of critical importance" to England.

He also said he hadn't been invited.

"Just as the bicentennial helped this nation get past Watergate and Vietnam, the wedding will bring their nation together despite their recent economy problems and racial troubles," he said.

"It gives the people a chance to express their affection for an absolutely wonderful couple, and through them to express their pride in their nation as a whole. It might just be of critical importance."

Lady Diana decided to carry in her bouquet a sprig of myrtle grown from a sprig carried in the bouquet of another 20-year-old royal bride, Queen Victoria, whose marriage in 1840 was one of the happiest in the annals of the House of Windsor.

A palace spokesman said that since the engagement was announced. Charles and Diana have received 100,000 letters and 5,000 presents, telegrams and other expressions of good will.

Most members of the royal family yesterday attended the christening of the daughter of Princess Anne, in the private chapel of Windsor Castle. Queen Mother Elizabeth was there, dispelling doubts her leg ulcer might keep her from the wedding.

Absent was Lady Diana, who was busy making last-minute arrangements.