London; A contrast of styles: prairie, Caribbean, New Edwardian

The Princess of Wales must be weary of all the copycat versions of her own fashion look trotting about the streets of Britain. This spring will make it worse. All the fashion designers are onto her pie-frill neckline, the swathed waist over midcalf skirt, the dashy sailor dress. And now, as she appears in maternity tents, even that style has caught on for women of every age and life style. The smock dress, often worn over contrasting underskirt, is a winner.

The only escape for an independent-minded fashion watcher is to go into the gypsy-Caribbean frills that Diana has not yet embraced. Of Britain's four major fashion stories for spring, that one has still to be explored by the royal trend-setter.

While the prairie front porch may not seem a natural habitat for the Princess of Wales, it is relevant in spring fashion. The girls who wore blue jeans out in the corral last year are now wearing their denim (and more likely their tablecloth checks, madras, and pin-striped cotton) in the form of a frilled mid-calf skirt that has the air of a long-ago front porch afternoon. With the frilled skirt - they're everywhere, from around (STR)10 ($18.20) and up - you may wear a snug-fit top with poufed Edwardian sleeves, a sassy peplum.

Cinching it all in is the huge and demanding waspie belt; some of these come eight inches wide. Though they're around at all prices, you can have one created just for you by Deliss in Beauchamp Place for a cool (STR)135 ($245).

The fun with these New Edwardian outfits is to mix textures and weaves. Caroline Charles, who's doing lots of clothes for the Princess of Wales these days, designed a freshly chic spring collection teaming a herringbone mid-calf skirt with a striped peplum jacket - all in chestnut and white cotton. All the popular chain retailers are shouting down the telephone for more of ''these prairie outfits,'' and girls who've never been farther west than Cardiff imagine themselves swirling down a western trail with Robert Redford.

Sailor looks appeal to British fashion watchers more than to most, so when St. Tropez relaunched the matelot fashion last summer, Britons took to it as a sailboat to water. Middy tops are fast vanishing from the services' surplus stores, and the knitwear men are scudding great sailboats across the fronts of their cotton made-in-Korea sweaters.

A serial like ''Brideshead Revisited'' has a marked effect on fashion here, but not always in the way expected. It's not Julia's clothes that have the girls reaching for their pound notes - it's Charles's. The argyle knit pullovers he wears are copied in all the women's shops, and a fresh updating from a Scottish mill has them in mouthwatering mixes of pink, khaki, and lemon yellow, all knit in pure cotton.

It's interesting, too, that three big serials on TV at the moment are featuring turn-of-the century clothes, with lace-trimmed blouses and sweeping full skirts. They're ''King's Royalty,'' ''Fame is the Spur,'' and ''Nancy Astor.''

Contrariwise, the minidress is also back, but it must now be a long-torso one. This long body with hem frill softens the dresses in many collections and gives a young, pert air to the miniskirt. And the strapless dress, long or (for holidays) short in typical print cotton, is having its greatest comeback since the days of Lana Turner movies.