Theatergoers in this town believe, with some reason, that their dramatic stage is the envy of the Western world. But Londoners have long suffered from an inferiority complex over one branch of the art - the musical.

As the Daily Mail put it the other day: "Leave the musical to the Americans. The British can't do it. To amateur, too ill-organized, too lazy . . ."

That is, until last week. With "Evita," a rock-pop drama more or less based on the life of Evita Peron, London is now convinced it has made the big leagues.

Months before the opus by Andrew Lloyd Webber and Tim Rice opened at the Prince Edward here, a brilliantly orchestrated press was celebrating a new-found British talent. The Sunday Times color magazine devoted seven pages of color pictures and text by lyricist Rice to "the most heralded musical of the Seventies."

The morning after first night was a triumph of self-fulfilling prophecy for Fleet Street.

The Daily Mirror and the Mail put the little-known star, Elaine Page, on their front pages under the headlines like, "Hurray for Me, I'm Evita" and ". . . Superstar"

The Daily Express cried, "There'll never be another night like it."

The Mail sung the nationalist theme with characteristic precision. "It is history being made," the paper said. "London has seen nothing like it, and it was actually born here."

The more serious critics were, for the most part, no less rapturous than the tabloids. The Sunday Times assigned two, John Peter, concentrating on the drama, declared:

"As a piece of stagecraft, no, stage art, this is one of the very best musicals London has seen in years."

His music colleague Derek Jewel declared the score was "a masterpiece" and the whole affair "quite marvelous modern opera."

Only the Sunday Telegraph's man thought that Rice and Webber had unhappily compromised their 'pre-packaged smash hit."

Francis King said it was a pity that Webber and Rice could not decide whether the bar girl who became Argentina's political saint was "a corrupt, scheming bitch" or "a misunderstood woman genuinely concerned for the underprivileged in her country."

But this shrewd shaft was buried in the chorus of acclaim. "Evita," say its exuberant press agents, is heavily booked until October and will be brought to New York next year.

For this seasoned Broadway and West End viewer, however, the pastiche of rock, tango and ballad with its mixed media deployment of drama, music and movie screen falls short. Its derivative quality is fixed before the curtain rises. It is covered with vaguely Latin American murals whose symbols - general on heap of skulls - are borrowed from Orozco's frescos at Dartmouth and sinister pastels from Diego Rivera on almost any Mexican wall.

On stage, things are little better. To bolster the frequently unconvincing action, a huge screen runs newsreel clips of the real Evita and Juan Peron.

This device was genuinely integrated in Tom Stoppard's brilliant "Jumpers."

A rock singer, David Essex, is gotten up to resemble Che Guevara and sings cynical commentary on Evita's rise from bed to boudoir. But the lyrics Rice has given him lack bite ("You let your people down, Evita/You were supposed to be immortal/But in the end you could not deliver").

Anyway, Brecht and Weill were their first in "Three Penny Opera," and without any fudge in the center.

Indeed, Rice's lyrics are frequently painful and all too audible on a stage loaded with hidden microphones. One number compels poor Elaine Paige to sing, "So Christian Dior me . . . so Lauren Bacall me - anything goes."

Compare those barbaric verbs with Cole Porter's ingenious contemporary superlatives - "You're a Bendel bonnet, a Shakespeare sonnet, you're Mickey Mouse" from, of course, "You're the Top" in "Anything Goes."

Webber's music is more bearable, and the show does have a couple of amiable ballads, including what is supposed to be the stopper, "Don't cry for Me Argentina."

But a musical drama with sung recitatives won't work unless the music characterizes personality - a rule as old as Mozart and new as Menotti.

Elaine Paige, out of Bognor Regis and West Hampstead, is something elst, a pocket-sized Ethel Merman, vital, compelling, a triumph of art over matter. How far she really can belt her songs won't be known until she is deprived of the electronic support in the Prince Edward. Her age is variously given as 25, 26 and 30.

Her treatment of "I'd Be surprisingly Good for You," that line with which she picks up Peron, is a surprisingly subtle blend of romance and come-on. But oddly enough, she has been dressed and directed to play in a markedly unerotic fashion.

The British promoters of "Evita" took no chances and hired American Harold Prince ("Company," "Cabaret," "Fiorello," among others) to direct. He relieves the tedium with some inventive bits - Evita as a prostitute, standing spread-eagled against a padded, mattress-like revolving door, singing, a parade of clients on their way: an upper-class ensemble, limp-wristed, in evening dress, gliding across the stage as an undifferentiated, decadent mass.

But even Prince borrows from Prince with a dance of street toughs straight out of his "West Side Story" and a placard-waving crowd from his "Pajama Game."

"Evita" may make a barrel, both here and in New York. But the Big Apple's reputation for unmatched musicals is not likely to be threatened by the goings on in the West End.