Maybe it's not fair to compare Washington's theater activity to London's. London had a formidable head start, and it's a much larger city.

No elaborate contortions are required to see three plays a day here. On almost any weekday, you can begin with lunchtime theater at 1:15. That's likely to last about an hour. Then it's a matinee at 3 - probably on the West End or at the National Theatre. After supper, playgoers can get their evening fix on the West, East, North or South Ends, at a variety of established and experimental theaters.

Nevertheless, there are some theatrical institutions that thrive in Washington more than in London. Dinner theaters. Sunday matinees. High ticket prices. Free programs. Standing ovations. If the capital of the United States really has all the international artistic ambitions that have been ascribed to it, ultimately it must bear comparison of its theater with the theater in the capital of the United Kingdom. Why not the best?

So here are some very carefree observations of the similarities and the differences:

The British National Theatre is hard to miss in its vast new home on the south bank of the Thames. Washington's closest equivalent to the National is not the equally visible Kennedy Center, however. The Washington institution that most resembles the National Theatre is Arena Stage.

Both institutions maintain three active stages of varying sizes and shapes. Both maintain a resident company, though the National's, with almost 100 actors on the payroll, is an army compared to Arena's little unit. They are equally serious about artistic aims. They mix classics and modern revivals and new plays. While they are not outre , or - to be British about it - on the "fringe," they are not afraid of dark, challenging plays. They attract large audiences, but the box office does not rule their roosts.

Of course their financial resources and hence their scope of activities are hardly comparable. This month I saw two shows at the National - a dreary "Macbeth" with Albert Finney and a simmering but not quite sizzling revival of "Plunder," a '20s farce by Ben Travers. During June at the National, you can also see "Brand," "The Cherry Orchard," "The Country Wife," "American Buffalo," "Don Juan Comes Back From the War" and new plays by David Hare ("Plenty"), Alan Ayckbourn ("Bedroom Farce"), Wilson John Haire ("Lost Worlds") and Ron Hutchinson ("Jews/Arabs").

They won't all be triumphs ("Macbeth" is the raw proof), but individual failures can easily be subsumed into the staggering strength of the whole enterprise.

Two samples of National work are coming to Washington next season. Arena will present a new production of the English version of Odon von Horvath's "Tales From the Vienna Woods" introduced at the National last season. And the Kennedy Center plans to present the National's touring production of "Bedroom Farce."

It's paradoxical but true that Arena's choice to do its own version of a National-bred play resembles National decision-making more than the Kennedy Center's booking of a genuine National production. The National may send its productions out on commercial tours, but it does not serve as a booking house for other commercial productions. This is one of the primary differences between the National and the Center.

There are others. Generally speaking the Center and its adjunct, our own National Theater, resemble the London West End (and the New York West End, Broadway) more than they resemble the British state theaters. The Center, with few programming subsidies, must sell lots of tickets to fill its large theaters, just like the West End. Stars and their vehicles tend to dominate. Plays with intellectual substance tend to be talk shows, lacking the theatrical drive that a more interesting (if smaller) space could provide. One of the current West End shows, Alan Bennett's "The Old Country," fits the above description well and therefore might fit the Center well. It even came with a favorite Center star, Anythony Quayle, on the day I saw it.

The Royal Shakespeare Company is Britain's other major state-supported theater. Considerable stretching is required to find a Washington shadow of the RSC, but the only obvious, if insufficient, candidate is the Folger Theatre Group. Both groups concentrate on Shakespeare and new plays. The physical facilities of the RSC are not as formidable as those of the National, just as the Folger's facilities are minimal compared to Arena's. But the importance of both the RSC and the Folger surpass their physical limitations.

Of course the accomplishments of the RSC are exceedingly more exciting and better endowed than those of the Folger. The RSC maintains two stages each in two cities, London and Stratford. The buildings may not match the National's, but the productions definitely do.

Perhaps the most riveting stagecraft in London is at the Aldwych Theatre in the RSC production of "Coriolanus," directed by Terry Hands with the charismatic Alan Howard in the title role. The black and white design of the stage is not far from that of "Macbeth" at the National, but "Coriolanus" actually emerges looking like a much more complex and powerful play. Howard is tough, terrifying, regal, irrestible.

Usually listed third in any discussion of London theaters is the Royal Court, patron of new playwrights and, again, an older and richer version of a Washington institution, the New Playwrights' Theater. Snoo Wilson's "The Glad Hand" at the Royal Court, which recently closed there, manhandled obscure American motifs in a rather enervating fashion.

A more characteristically American play was seen at the Open Space, Charles Marowitz's outpost of the avant-garde. Tom Thomas's "The Ball Game," a savage sitcom about some residents of Pittsburgh on the night the Pirates won the Series, is full of desperate energy and cynical laughs. Not all of the Americanisms were down pat in the acting, however.

Two ventures into lunchtime theater were more rewarding tastes of new playwrights' work. At the Almost Free Theatre (best known here for its hatching of Tom Stoppard's "Dirty Linen"), "Distant Encounters" was an intriguing adaptation of three sci-fi stories by Brian Aldiss. And the Astoria Lunchtime Theatre offered John Baliol's. "After the Ball," an absorbing look at two rather charming mental patients. ASTA's new soap opera is the closest counterpart in Washington to these lunchtime stages.

Of course some London theatrical institutions seem made just for London - "The Mousetrap," for example, in its 26th year on the West End. Though Olney gives us sporadic doses of Christie - such as its current "Black Coffee" - there's nothing in Washington that can really compare with "The Mousetrap" at age 26. London would not be London without "The Mousetrap."

Generally, however, the outlines of a nascent London theatrical scene can be detected in Washington, if one squints. Indigenous activity, of course, is what makes any city worth visiting, and no one expects Washington to be another London. Still, it would be nice if there were three diverse, professional plays available in Washington any day and every day. Maybe, someday, London tourists can come to Washington for the theater.