IF ED BERMAN accosts you on a sidewalk and offers you a once-in-a-lifetime shot to buy a fragment of "Hamlet" -- personally autographed by William Shakespeare -- you should not feel insulted.

It isn't that he thinks Washingtonians are unusually stupid. He has worked this routine outside London's National Theater, too, armed with a living, breathing William Shakespeare and an 8-foot-high, inflatable Elsinore Castle. And to hear him tell it, those sawed-off pieces of "Hamlet" went like hotcakes.

Carving up old copies of "Hamlet" is only a sideline for Berman, a driven (and driving) man who meets the press with the aura of a guerilla general down from the hills.

Berman is the founder of Inter-Action, a London arts collective that performs plays on double-decker buses, practices and teaches something called "the Game Method" (using children's games as a means of instruction and therapy), and runs a lunch-time theater, a children's theater, urban farms, playgrounds and an alternative school for truants. And, oh yes, presents the latest plays of Tom Stoppard.

Inter-Action has also, just now, created the British-American Repertory Company (BARC), which, under Ed Berman's direction, will open Stoppard's "Dogg's Hamlet Cahoot's Macbeth" at the Kennedy Center Tuesday night.

BARC -- half of its 16 actors are British, half American -- was conceived, frankly, as a way around restrictive union rules in both Great Britain and the United States that keep each country's actors out of the other country. The basic exceptions are "international stars" (like Maggie Smith, who will star in Stoppard's "Night and Day" at the Kennedy Center in October), and a few recognized "unit companies" (like the Comedie Francaise and the Royal Shakespeare Co.).

"I tried to bring the Dogg's Troupe show [his children's theater company] here many years ago," says Berman, "and I was stopped. Dogg's Troupe wasn't special."

The rules that govern the use of foreign actors here are set down in the basic agreement between Actors' Equity and the League of New York Theaters (an agreement whose terms also cover the Kennedy Center and the National and Warner Theaters in Washington). To be certified as an international star, an actor or his prospective producer must submit documents attesting to the "current widespread acclaim and international recognition of the alien," and Equity, if persuaded, will then support the actor's application to the Immigration and Naturalization Service.

An approved "unit company" must come here with a repertory of at least two plays, and cannot remain in any city for more than 20 weeks. Alan Ayckborun's "A Bedroom Farce," brought here last year by Britain's National Theater, was a special case. The company had only one play on its repertory, but agreed with Equity to replace its eight-man English cast within five months (in specified phases).

There is a third exception, less often used, for an actor providing "unique services" -- services that, in Equity's judgment, cannot be provided by any of its members. "A 3-foot-8-inch sword-swallower would probably qualify," says Willard Swire, Equity's associate executive secretary.

When Berman directed Stoppard's "Dirty Linen" here at the end of 1976, he was forced to re-cast the show with American actors. He describes the production as "perfectly good and acceptable," but believes the play (specifying an assortment of English regional and class dialects) suffered from the shift. What's more, he believes American actors suffered, too.

If the original British company had been allowed to come in and open that show," Berman says, "and if American actors [as eventual replacements] had been allowed to become conversant with the accents and regional traits, I think we would have added a year's worth of life onto the show."

"In theory," says Swire, "he might be perfectly correct. In practice, I'm not so sure. There have been plays that were hits in London and died in America for reasons that really had nothing to do with the casts. There are other plays that have run for years with American casts."

British Equity, says Swire, not only has a stricter policy of excluding American actors but more influence with British immigration officials. "Speaking for myself, I'd like to see an open-door policy in both countries," he adds.

When a British production has to be re-cast here, or vice versa, accents are the first point of stress. But problems of accent, according to Berman, lead to problems of timing and feeling. "It's the innards, what's in the marrow of the characters, often expressed through accent," he says.

An original work "transferred by rote," says Berman, "cannot be the same . . . If I were trying to cast an American play now in London, I might find six or eight perfectly good British actors. But if I had a character role, a cameo part that had to be a downeaster, I might not be able to find an actor to do that. And it's extremely difficult to get black Americans played by actors in England. The cadence of the language is totally different."

"A single bad portrayal can ruin a production," says Berman.

He also sees BARC as a bridge between two different acting traditions -- the classical style of the English, radiating technique, and the "muscular, energetic" style of American actors.

To arrive at its final 16, BARC interviewed 875 actors in both countries. The goal was to create a company that would be utterly indistinguishable by nationality, says Berman, and he proudly points out that the six Shakespearean roles in "DOGG'S Hamlet Cahoot's Macbeth" happen to be played by American actors.

So far, BARC has performed in half a dozen cities across England and Scotland, and in Holland for two weeks. The original plan was to go abroad with a repertory that would include, in addition to "Dogg's Hamlet Cahoot's Macbeth," a bill of one-act plays by Wolf Mankowitz and Berman himself. But Berman decided to drop his own and Mankowitz' plays in favor of Stoppard's "Dirty Linen" for the coming tour of the United States and Australia. (Washington will see only "Dogg's Hamelt" because "Dirty Linen" was done here 2 1/2 years ago.)

"If you've got the best running back in the world," says Berman, explaining the substitution, "you're not going to pass on first down."

"Dogg's Hamlet" is, in his considered opinion, "the funniest play Stoppard has ever written." During the burlesque version of "Hamlet," a "wall of laughter" occasionally renders the actors inaudible, says Berman (but they are not allowed to stop acting), and the "Cahoot's Macbeth" half of the evening (dedicated to Czech playwright Pavel Kahout) is "high comedy" with a "thrust of fear and menace juxtaposed against typical brilliant humor."

In keeping with BARC's policy of strict equality between the nations, its leader is a half-and-half product, too. (His application for British citizenship became the springboard for "New-Found-Land," a short Stoppard play performed here with "Dirty Linen.")

Ed Berman grew up in Maine and studied biblical archaeology at Harvard. He also wrote plays there, and claims the distinction of having had a Hasty Pudding Show script rejected on grounds of bad taste.

A Rhodes Scholarship brought Berman to England in 1965, but he was bounced out of Oxford after a week, he says, and then went to Istanbul to study Ataturk's use of school textbooks to pound the spirit of nationalism into his people.

Unfortunately, Berman got a more vivid look at the spirit of nationalism than he had bargained for. At home one Sunday morning in his scenic apartment overlooking the Bosporus, "I was sitting on my patio reading my Sunday newspaper when this guy came up behind me and tried to club me to death."

His assailant was a neighbor -- a soldier home on leave with his family, who must have been a "super-patriot," Berman theorizes, going after "the nearest handy American" at a time when the struggle over Cyprus had made Americans generally non grata in Turkey.

X-rays showed a blood-clot on Berman's brain, and doctors gave him a year to live. But as things worked out, he had only to deal with a year of intense, recurring pain, when "every waking minute was unpleasant."

The soldier was never charged. "I think he's a national hero," Berman explains.

Back in London, Berman submitted a play to the International Theater Club in Notting Hill, a West Indian immigrant area, and was soon named the theater's "resident dramatist" (sleeping, appropriately, on the rehearsal-room floor).

A year later, "I had moved through most of the jobs in theater and had three of my plays done," he says. But Berman was determined "to make the techniques of theater more relevant to social problems" -- an impluse that led him (in 1968, as he was getting adjusted to the possibility that he might, after all, go on living) to found the Inter-Action Trust.

The majority of Inter-Action's activities sound about as profitable as Chrysler's latest line of luxury cars. But the collective's 40 full-time members receive only 14 pounds a week apiece, plus bed and board in the group's own Kentish Town dormitory. And Berman has managed to make Inter-Action's earnings -- from ticket sales, book sales (volumes on battered women and football hooliganism, as well as plays), and classes (in acting and the Game Method) -- cover 70 percent of its expenses, Berman says. Government and foundation grants cover the rest.

And just as Joseph Papp's Public Theater has "A Chorus Line," Inter-Action has "Dirty Linen," which was first launched at the group's Almost Free lunch-time theater in Soho and is now in its fourth year at the small Arts Theater on London's West End, pumping funds back into the parent conglomerate.

Berman thinks Inter-Action has a future on this side of the Atlantic, too, and calls BARC a "holding action, a Trojan horse, a flagship -- anything trite will do."

Since he has made a career of belittling institutions like the Kennedy Center, Berman's appearance there has a certain fox-in-the-henhouse quality. But if all goes well, he hopes to be back next year, armed with a few of his weirder acts and anxious to prove that his bite is worse than his BARC.