For 25 years or so, Joseph Papp has been hinting that the British have a thing or two to learn about Shakespeare. Now he suggests that Gilbert and Sullivan too, have not been perfectly treated in the home country. Can war be far off?

The latest American affront is New York City's terrific outdoor "pirate ofPenzance." Except for a skyful of ominious rainclouds, which politely contained themselves through Monday night's opening in Central Park, all the other witnesses to the event (including Mayor Edward Koch and a host of notables) gave it the thunderous reception it deserved.

Papp and Co. have long been known for casting against type and getting away with it. Well, they have done it again, with -- going from least to most startling -- George Rose, Patricia Routledge, Kevin Kline, Rex Smith and Linda Ronstadt.

There are a few things about the musical theater to which Ronstadt could profitably devote further study (acting come to mind) but casting her as Mabel, the "pirate's" ingenue, was a stroke of genius. Dressed from collar to toe in proper Victorian women's wear and with her hair rather straighter and less assertive than usual, she seems to have purged los Angeles from her soul in favor of London Bridge. And she sings her part beautifully, revealing a techinical virtuosity and high range that haven't often been heard in her records. Sure, she could put a little more life into her dialogue; but since there are only about six lines of it, who cares?

Ronstadt is the drawing card, but this production has losts else to recommend it. Normally (which means, when you are talking G & S, the D'Oyly Carte version) the singing itself is the essence of a production and only one character per show, usually an older male, is allowed much comic elbow room. Here, with Wilford Leach directing, the actors don't just stand there politely and sing; they act, they interact, they dance, leap, pounce and fall on their various hindquarters. In short, they play pirates as theater, not as operetta -- that bastard form of entertainment that can loosely be defined as compromise between opera and musical comedy in which the virtues of both are sacrificed simultaneioulsy.

The visual hijinks get a poweful assist from the scenery, especially a jolly little roger of a pirate ship that wheels on and off stage in Cat 1. and Graciela Daniele's burly beltsy choreography fits rith in with the rest of the action, thanks to the distinctive dance roles she gives the individual members of the chorus. (The one notable error on which she and Leach have collaborated is a Max Sennett-inspired crew of Keystone Kops whose unmitigated feebleness gets tiring in a hurry.)

The story is that of Frederick (Rex Smith), whose father orders him apprenticed as a pilot at an early age. Through phonetical confusion becomes an apprentice "pirate" instead.This grotesque mistake is committed by his nurse Ruth (Patricia Routledge), who, in her shame, has stuck by his side ever since. But as the story begins, Frederick is about to reach age 21; when he will be free to leave piracy and pursue an honest life.

As much as he loves his fellow pirates as individuals, he tells them, he must dedicate the rest of his yers to their extermination, it is a matter of duty. On the other hand, while one last half-hour of his piracy career remains, he feels obliged to mention a few small failings of procedure that explain, he thinks, why these pirates have never been able to make a dishonest living. Their problem is that they are too nice. "You make a point of never attacking a weaker party than yourselves," he says, "And whenever you attack a stronger one, you get thrashed."

There is a tendency -- woefully encouraged by the critical profession -- to assume that a story this footloose and fancy-free must be utterly without serious import. Nothing could be further from the truth. The basic allure of "Pirates" comes from something more than the brilliance of Gilbert's wit and the richness of Sullivan's music. Underneath it is a gorgeous reaffirmation that all the fractious elements of mankind share a common desisre (however repressed) to keep our commitments and play fair that this bond exists even as we take each other hostage, aim missiles at each other's capitals and grab each other's natural resources.

but on to matters more frivolous and immediate. For example, George Rose -- a recent visitor to Washington as Captain Hook in "Peter Pan" and as Rex Harrison's valet in "The Kingfisher. "He is an absolute cut-up in a nightshirt and stocking cap, singing, "I Am the Very Model of a Modern Major-General." And Routledge, a star of British musical comedy, is just as predictably wonderful as Ruth, who vainly tries to keep young Frederick from ever learning that there are women younger or more beautiful than she.

As for Ronstadt, she makes a knockout entrance with "poor, Wand'ring One," followed by a dazzling "ha-ha-ha-ha" cadenza that had the opening-night audience justly wowed. But while this music comes unexpectedly form her lips, it is no suprise that she has one of the best voices around and Rex Smith (of "grease" semi-fame) is an excellent choice to play opposite her; their mid 20th-century singing styles are a consistent, unjarring way of distinguishing the young romantic lead from their elders.

The real surprise of the show is Kevin Kline (last seen in "Loose Ends" at Arena Stage) as the Pirate King. Brandishing his sword with such abandon that his fellow pirates have to run for cover, trading blows with the conductor and even knocking the batton from his hand, Kline is a mad union of conscience and criminality. He shows whole new possiblities, including the possibility that he may be one of the most versatile young actors in the American theater.

All in all, "Pirates" is an inventive, truly funny production that will fall happily into place with the other memorable high points of the New Shakespeare Festival's quartercentury of existence. But one thing is terribly amiss, and forgivable only because it is probably incurable. In the open air of the Delacorte Theatre, the amplified sound that come from the chorus -- while highly agreeable -- only occassionally resembles the English language. (The same was true Monday night of several Ronstadt solos, a problem that seemed to result from some combination of vague enunciation and a defective microphone.) It would be a great good fortune if the show could move indoors -- which may happen after its curren run ends Aug. 24. At all costs, it should be recorded.

This is a gift horse, however, so it shouldn't be looked too closely in the mouth. Tickets are free (audiences line up at the Delacort each afternoon for the evening show), continiuing a tradition that has had Papp and several generation of New York City politicians at each others'throats.

A feud between Papp and the current city administration has resulted in the absence of free Shakespeare this summer. In the meantime, Gilbert and Sullivan are a splendid substitute. Central Park has never seemed safer at night.