The `Prime Mimicker' of the British prime minister

If anyone can claim to be Margaret Thatcher's double, it is surely actress Janet Brown. Her theatrical career has been distinguished by her uncannily accurate impersonations of celebrated people. Of these her ``Mrs. T.'' unquestionably tops the bill. This versatile artist tells the story of her life in an autobiography just published in Britain under the unlikely title of ``Prime Mimicker'' (Robson Books, London, 8.95). DELICATE hints of her Scottish origins still linger in Janet Brown's speech -- when she uses her own voice, that is.

But this entertainer/comedienne/actress/impressionist/television personality/cabaret performer is master of many voices (not to mention faces).

One moment she may be intoning the stretched and gravelly vowels of American actress Carol Channing. Next she is tearfully grasping at the catch-in-the-throat drawl of ``Dynasty's'' Sue-Ellen Ewing. As suddenly, she is stentorian in the persona of Barbara Woodhouse, the British dog trainer extraordinare: ``Si-TT! Sta-YY!''

Ms. Brown literally thinks herself into her different characters, themselves larger than life, such as Marlene Dietrich, Katherine Hepburn, Dolly Parton, and the Queen of England. One of her latest acquisitions is the squeezed, high-pitched, vocal contortionism of Liverpudlian actress Julie Walters, star of the British stage play ``Educating Rita.''

There is one voice, however, that quite naturally dominates all others. It is the voice (and face) that has brought her a more widespread, peculiar kind of celebrity in Britain than ever before in her career, and a growing reputation abroad. It has also brought her a considerable amount of work as an unpredictable after-dinner speaker.

That one is the so indelible, so deliberately authoritative, so low and slow voice of Margaret Thatcher, Prime Minister of Britain. A London cab-driver (as a TV performer, Brown relies on their criticism) told her:``I think you've made her famous!'' Brown laughed gleefully during our interview as she recalled the quip. There may be an element of truth in it.

The first time she did ``Mrs. T.'' was when that redoubtable lady was elected Leader (in Opposition) by her fellow Conservatives in 1975. This initial impersonation was an unscripted, last-minute performance for an early-evening TV show. It was immediately noted by the press. With increasing sophistication and wit -- and remarkably little unkindness -- Brown has developed it into Live Cartoon No. 1 of Britain's first female prime minister.

Brown's fame has come -- as her just-published autobiography delightfully chronicles -- at the peak of a long, varied career on the boards going back to the 1930s. Then, as a stage-struck thirteen-year-old, she won a talent-contest in Glasgow by impersonating Shirley Temple and Mae West and joined a touring show. Her concerned parents finally had to fetch her home -- though by then, the theatrical die had been irretrievably cast. She was soon entertaining the British troops of World War II with her comedy routines on radio, stage, and television.

One of the hazards of Brown's profession as an impressionist is that sometimes people get confused -- or pretend to -- about her real identity.

``I'd go to buy a rail ticket,'' she says in her book, and the clerk would say: `` `Well, Maggie, where are you off to today? Aren't you supposed to be in Germany?' I'd hail a taxi and, depending on the driver's politics, I would hear, `Jump in, Maggie,' or `I'm not giving you a lift.' It was all good-natured fun and I enjoyed it.''

Her impressions of Mrs. Thatcher have led her into some strange waters. One time she addressed a congress of socialist (vigorously anti-Thatcher) trade unionists. They chanted, ``Get off! Get off!'' until they realized it was a send-up. The laughter then became as loud as the insults had been -- except for one ``stony-faced woman'' who came backstage afterwards to say: ``I'm sorry, but I can't shake hands with you. . . . You're too much like the real thing.''

Politicians are not known for shunning publicity, favorable or otherwise, so it should not be surprising that since 1975 Thatcher One and Thatcher Two have corresponded affably and met on several occasions. Brown says in her book that Mrs. Thatcher's TV image had suggested to her, before they came face to face, that Thatcher was ``a rather aloof, rather cold lady.'' So Brown was surprised to find she was actually ``this woman with this great warmth,'' who above all seemed ``motherly.'' ``I feel this warmth that she has isn't shown by the media,'' Brown added.

One time she asked the Prime Minister ``how she continues a line in [a debate] in the Commons when there is so much barracking. She replied, `Well, you know, if you're standing in the kitchen with the radio on and you were saying something, you just continue regardless. It's just like a radio to me, I ignore it!' ''

One of Brown's favorite studies is the behavior of politicians. She enjoyed recently watching Thatcher's skill with a particular television interviewer.

``He was very gentle but very persistent. . . . He asked her a question at one point. . . . She's become so sharp! She said to him [and Brown's own voice disappears into the Thatcher-tone] `You leaned in there, Mr. Dimbleby. . . . Did you think you were going to ca-atch me ou-t]' The interviewer burst out laughing.''

Perhaps Thatcher learns something about quick repartee from Brown. Who knows? All British politicians begin, sooner or later, to emulate their impersonators.