Margaret Thatcher, the first British minister with apple-blossom skin and a superb lace collar, walked up the ceremonial stairs of the British Embassy last night with her guest, President Reagan, who radiated confidence and enthusiasm as he strode towards his lemon broth soup and quail pie and the rest of a fabulous supper.

Thatcher had arrived 45 minutes earlier in a procession of a dozen limousines for one of the major events of her official visit to the capital, in which she has stressed Anglo-American ties and the conservative philosophical bonds that relate her to the new American president.

Denis Thatcher, the prime minister's husband, lit a cigarette at the first feasible break, comforting several additional sinners who had thus far refrained, and as President Reagan arrived, the foreign secretary, Lord Carrington, said, "Pencils out, pencils out," to reporters who might need prompting.

For 40 minutes before the president and his wife, Nancy Reagan, arrived, Margaret Thatcher stood on the little black-and-white marble pavement at the top of the double wrought-iron-railed Georgian staircases greeting a hundred guests of the highest distinction or official rank. There may have been particular warmth in her reception of Secretary of State Alexander Haig; Senator Paul Laxalt (R-Nev.); Barbara Walters of ABC television ("You're doing me tomorrow morning"); former Ambassador Walter Annenberg and his wife, Lee, the chief of protocol.

The only one she kissed, however, was Pamela Harriman, wife of the grand ol' man of the Democrats, Averell Harriman.

Pamela Harriman, inspired by this beginning, kissed everybody else in the receiving line.

Thatcher's toast to the president was particularly long, warm and enthusiastic. After a number of comments about mutual values, she said a president might encounter "times of rough water . . . when only you can make a decision . . . and you need two-o'clock-in-the-morning courage."

Only one thing, she said, will sustain a president at such times: total integrity.

"When those minutes come, we here in this room have in you total faith."

She also made some light laughter-provoking comments quoting Dickens and Thoreau, and nodded toward Bob Hope, whom she said had doubtless left his native England "because he thought American gold courses were better."

The president responded warmly, complaining mildly that "you are a hard act to follow . . . I do know something about two o'clock couage and I know you have already shown it."

He too spoke of "civilized ideas, the rights of man, the rule of law, and parliamentary democracy" as special glories of Britain.

"You and I have had our share of dire predictions . . . from those who preach the supremacy of the state," but these are merely "a gaggle of bogus prophecies," and he said both nations should be planning for a world in which these bogus prophecies were merely a bad memory.

After dinner, guests sat about their tables or wandered through various reception rooms or listened to music.

Sen. Claiborne Pell (D-R.I.), the ranking minority member of the Foreign Relations Committee, set off to find a cigar and others, more solicitous for his pleasure than his health, got cigars for him so that he wound up with a pocketful.

Other congressional leaders included Sen. John Tower (R-Tex.), chairman of the Armed Services Committee; Sen. Howard Baker (R-Tenn.), the majority leader; Rep. James C. Wright Jr. (D-Tex.), the other majority leader; and Rep. Clement Zablocki (D-Wis.), chairman of the Foreign Affairs Committee. John Shaheen of New York, chairman of Shaheen Natural Resources Co., was probably the only guest who had known Reagan as a boy in Tampico, Ill.

Thatcher, wore a long dress of possibly taffeta, (the favorite fabric of Shakespeare) not flame-colored but black, with a great lace collar and a black rose made of cloth, caught by a cluster of diamonds.

Lady Carrington wore a dove-gray dress of thin material embroidered from the hem up to her knees with a pale elegant design of marsh grasses such as one sees in the Norfolk Broads.

To aid her during the receiving line, the prime minister was given a small, delicate table before which she stood and on which she set her purse and a drink which required 40 minutes to finish, what with greeting everybody.

The British ambassador, Sir Nicholas Henderson, strode here and there making people welcome. His wife wore a summery white dress with a pink camellia in her hair.

Bob Hope and his wife were present, so was the whole Reagan Cabinet, and a notable set of corporation and institutional chief executives (Shell Oil, Dart, Occidental Petroleum, General Electric, Chase Manhattan, General Motors, Boeing and AFL-CIO).

Evangeline Bruce went through the line modestly and without ado, though she was particularly whammo and with-it. The hostess, Thatcher, was warm and deep-throated with all, no matter what they looked like, and needless to say, many admirable people are neither beautiful nor well turned out.