The gloom lifts from London's theaters

Suddenly the joint is jumpin'. Theatrically, London has come alive. A year ago many of the 40 commercial theaters here were dark. Even Agatha Christie's ''The Mousetrap,'' now into its third decade, began to look like serious theater. Among the few new productions, only Tom Stoppard's ''The Real Thing'' generated any real excitement.

But this year the gloom and doom befogging London's theater scene have lifted. There is a mood of cultural confidence in the air. Producers are producing, investors are investing, and, thanks to a drop in the pound, visitors are visiting.

And today, more than ever, it is profoundly evident that New York and London are becoming increasingly intertwined in theater. Inevitably, the hits seen in the West End cross over to Broadway, and vice versa.

Both ''The Real Thing'' and ''Noises Off'' are just about to spawn their Broadway versions. And such American-born plays as Christopher Durang's ''Sister Mary Ignatius Explains It All for You,'' the musical comedy ''Little Shop of Horrors,'' and Bob Fosse's ''Dancin' '' are all playing here. ''Cats'' continues to sell out in both cities.

Even Off Broadway theaters are becoming havens for newly imported English dramas, such as ''Fen'' and ''Top Girls,'' by British playwright Caryl Churchill. Perhaps symbolic of all this cultural exchange is the Old Vic theater. One of London's oldest and most historic houses, it has just been purchased by a North American, millionaire Canadian businessman Ed Mirvish.

And Britain's subsidized theaters - the National Theatre and the Royal Shakespeare Company - are increasingly selling their unique productions to American producers. Last year's Broadway hit was the spectacular Royal Shakespeare Company adaptation of ''Nicholas Nickleby.'' ''Poppy,'' another recent RSC production, has just moved into a West End theater here and is rumored to be Broadway bound.

And, most controversially, the new musical ''Jean Seberg,'' a product of a Broadway composer-lyricist team that includes Marvin Hamlisch, is just about to open at the Olivier Theatre. Many observers are questioning the role of the subsidized National Theatre (of which the Olivier is a part) acting as testing ground for an obviously risky American musical.

Such cultural cross-fertilization is generating a lot of talk, but presumably even more money. And so the trend continues. It is admittedly a heady time here, with new or new-to-London plays springing up all over town. For the theatergoer in search of tomorrow's hit or a uniquely British experience, it couldn't be a better season.

If one hasn't yet caught the by-now old warhorse productions of ''Evita'' and ''Cats,'' among others, well, they are still percolating along in the West End. There is already talk of a new musical from composer Andrew Lloyd Webber which has something to do with people on roller skates.

Then there are the inevitable revivals, which can often be dreary, but this season happen not to be. Most discussed here are new productions of Chekhov's ''The Cherry Orchard,'' starring Joan Plowright; O'Neill's ''A Moon for the Misbegotten,'' with Frances de la Tour; and Noel Coward's ''Hay Fever,'' with Penelope Keith, a hilariously haughty comedic actress whom even Americans should recognize from public television.

Two new productions of Shakespeare plays - often the best London has to offer - have opened. This time it's ''A Midsummer Night's Dream,'' at the National, and ''Tempest,'' by the Royal Shakespeare. Both are earning high marks.

The latter is a spectacular production - the audience broke into spontaneous applause during one scene change recently - featuring Derek Jacobi. Jacobi also stars in the RSC's ''Cyrano de Bergerac,'' which some are calling the performance of his career. Productions like these are seldom duplicated elsewhere.

But clearly, the big dramatic news is being made by the Royal Shakespeare with its production of David Edgar's new drama, ''Maydays.''

Perhaps best known in America as the author of the recent stage adaptation of Dickens's ''Nicholas Nickleby,'' Mr. Edgar is perceived in his native England as a top-notch playwright of serious political inclinations. His earlier work, ''Destiny,'' in which he chronicled the views of the political right, was hailed as one of the most significant political plays in recent memory.

Now the debut of ''Maydays,'' the first serious work to be given its world premiere by the Royal Shakespeare in its new facililties at the Barbican, only cements that reputation.

This is a far more ambitious play than ''Destiny.'' It details the beliefs of the radical left while incorporating the panoramic scope Edgar demonstrated in ''Nickleby.'' Spanning the decades from the end of World War II and the Labour Party victory right up to present-day disarmament rallies, ''Maydays'' is an audacious and sweeping look at the intellectual and social history of those years.

Edgar has crafted a balanced, intelligent, and coherent look at socialist ideals and their effect on people's lives, specifically on two British intellectuals and a Soviet dissident. He uses the quick-take scenes that he put to great effect in ''Nickelby.'' And the production, slick, arresting, and sophisticated, keeps the plot rip-roaring along.

''Maydays,'' understandably, has been winning a lot of praise from local critics. But then this is what British theater does best - intellectual, knotty problems thrashed out on stage. Anyone looking for emotion rather than passion, or for easily recognizable moral viewpoints - and one suspects that some American audiences might fall into this category - will be disappointed.

Edgar is careful to put his politics in the mouths of characters, but even so this remains a play of ideas and principles. Relationships take a backseat. And with the exception of the Russian dissident, played superbly by RSC actor Bob Peck, no character truly grabs the audience by the heart. Nonetheless, ''Maydays'' is a must-see here, for such political dramas do not often find fertile ground on the Great White Way.

But over at the National's three theaters perched on the South Bank, American interests appear to be holding sway. The just-opened musical there, ''Jean Seberg,'' is a largely Yankee product, despite its direction by Englishman Peter Hall. And a revival of George S. Kaufman's ''You Can't Take It With You'' is playing at the Lyttelton Theatre. In the National's smallest auditorium, the Cottsloe Theatre, David Mamet's latest play, ''Glengarry Glen Ross,'' premiered in September.

Critics and audiences here are responding well to this latest indictment of American capitalism - themes Mamet explored more successfully in his earlier ''American Buffalo.'' This play, a day in the life of five eat-or-be-eaten Chicago real estate salesmen - Mamet worked for a time as just such a salesman - certainly fits right into the author's traditionally bleak vision.

But at a recent performance, directed by Bill Bryden, who also directed the National's production of ''American Buffalo'' in 1978, one could not help wondering if approving English audiences found the spare, obscenity-laced dialogue truly provocative or simply a dramatized stereotype of the inarticulate , prone-to-violence American.

Better to watch the English portray themselves, and for this there is no better vehicle than the newly opened ''Pack of Lies.'' Based on the actual circumstances of some Soviet agents masquerading as a suburban London couple, ''Pack of Lies'' is less a tale of espionage than a faithful portrait of that meek segment of British society.

The play, written by Hugh Whitemore, who last explored England's version of suburbia in his play ''Stevie,'' succeeds largely because of its star, the excellent British actress Judi Dench. With her blocky, ugly shoes, droopy cardigan, and bouffant hairdo, Miss Dench is the archetypal 1960s English matron.

And yet she is much more. Despite an awkward dramatic structure that includes a number of solo narrative scenes, she infuses her character with a rich dignity that only grows more apparent as her secure little world crumbles under an awful deception. Michael Williams as her mild husband, Bob, is also careful never to make fun of his character. Together, they are a couple who by their own admission ''stand in queues and never answer back.''