London — GOING to the theater in the summer has its own delights, its own atmosphere. You spill out of the foyer onto the terraces in the interval, blinking in the sunlight. Part of you is still in another world: You are engrossed in ''The Government Inspector,'' or halfway through ''The Beggar's Opera.''
It seems unbelievable that the rest of the world is either hard at work in offices, or wasting time on the cricket pitch or the beach. You could be sightseeing, of course, but you have chosen instead the darkened auditorium and the fantasy, absurdity, or intensity of the play. Time is oddly suspended and you are conscious of a certain cultural superiority - of replacing the ordinary and mundane with extraordinary artistry, with flights of imagination, brilliant articulation, uncontrollable laughter. Or that, at least, is the ideal.
Of course summer theater also - ideally - involves visiting a beautiful place and experiencing a memorable occasion, and if possible it should be out of the metropolis. This is probably why the theaters outside London, which thrive most in the summer months, are to be found either in such historic showpiece towns as Bath, York, Edinburgh, Stratford, Oxford, and Cambridge, or, somewhat remotely, in the countryside - in the Lake District or the Scottish hills. Seaside resorts in England still have some big summer theaters, though many have now closed, and the old tradition of long-running major productions is giving way to a more varied format of briefer entertainments for summer visitors.
All told, England has 380 regional theaters. Recent research indicates that there are more of them now than in 1975, with 40 new ones opening or reopening in the period 1975 to 1982 and 18 closing.
By no means all regional British theaters open their doors in the main tourist season. But there seems little doubt that those that do not go ''dark'' in the summer offer, for the most part, good, sound British acting and directing of tried-and-true ''classics'' and a few new works by established playwrights.
To find less conventional work by new, young writers in the summer is harder, and involves visiting cities that are less popular as tourist meccas: Manchester or Sheffield, for example. The Playhouse Theatre, in Nottingham, is also active right through the summer, and is capable of adventurous theater, although this year it is rather more conventional as it builds up to its 20th birthday celebrations in December.
This year the tourist-in-search-of-summer-theater outside London and the industrial cities will be likely to encounter Shakespeare (plenty of Shakespeare), Chekhov, John Gay, Priestley, Osborne, Agatha Christie, Rattigan, J.M. Barrie, Emlyn Williams, Ben Travers, Brian Friel, and Alan Ayckbourn. To see all of these, of course, he might have to take a zigzag, up-and-down course for most of the summer, stretching from Chichester on the south coast to Pitlochry in the Perthshire Highlands, to Stratford in the Midlands, and to Scarborough on the east coast of Yorkshire.
As I write, many of the theaters contacted have still not completed plans for their summer programs.
The Chichester Festival Theatre, however, is perfectly clear about its plans. Twenty-two years old this summer, Chichester has consistently come up with an annual May-to-October offering of outstanding drama with star casts. Laurence Olivier was this theater's first artistic director in the '60s, which gives an idea of the place's ambition. The 1983 repertory season begins with Alan Bates in ''A Patriot for Me,'' (May 11 to July 2) the first major production of John Osborne's ''narrative drama'' about an Austro-Hungarian army officer blackmailed into spying for czarist Russia. The reawakening of interest in J.B. Priestley is celebrated by a production of ''Time and the Conways'' (May 25 to July 23).
Then Chichester is using its open stage as the leafy floor of the Forest of Arden, from July 13 to the end of September. This production of ''As You Like It'' is to be directed by Patrick Garland, who is Sir Laurence's latest heir to the throne as current artistic director. Patricia Hodge is playing ''fair Rosalind.'' She performed at Chichester two years ago in ''The Mitford Girls,'' and has starred with Ben Kingsley and Jeremy Irons in Sam Spiegel's ''Betrayal.'' ''As You Like It'' is a perfect summer play, and Chichester Festival Theatre, with its pleasant parkland setting on the edge of that beautiful small cathedral city, is everything a summer theater should be. To complete its season, Omar Sharif is to make a rare stage appearance in Rattigan's ''The Sleeping Prince'' (August 3 to October 1).
Chichester is one regional British theater that is so popular that early booking of seats is advisable. Matinees are on Thursdays and Saturdays.
In the past, quite a number of Chichester productions have gone to the West End of London. The Yvonne Arnaud Theatre in Guildford, Surrey - pleasantly situated on the tree-shaded banks of the little River Wey, sailed by ducks and swans - also sometimes initiates London successes. Top-ranking actors frequently perform here. From June 8 to July 2 this year ''Hobson's Choice,'' the north country comedy, is scheduled.
There is no doubt that first-rate British acting and directing can be seen outside the capital. And it can be seen at prices much lower than those in London theater.
There is, however, one honorable exception to this, and that is the Royal Shakespeare Company (RSC). With two homes, one in London and the other in Stratford-on-Avon, this world-famous company finds that market forces dictate that its most expensive seats in London should actually be (STR)3 cheaper than those in Stratford. In Stratford the best seats cost (STR)12.50. In London you can buy a seat as cheaply as (STR)2 - but the same money only gets you standing room at Stratford.
This year's summer program at Stratford includes two old favorites ''Julius Caesar''(opened Mar. 24) and ''Twelfth Night'' (opened April 14) and two less frequently performed plays by Shakespeare, ''Henry VIII'' (opens June 9) and ''Comedy of Errors'' (opens August 4). ''Measure for Measure'' will open in October. All Stratford plays run in repertory right through until January of the following year. Also in Stratford the RSC runs a small venue for less expected drama, ''The Other Place.'' It seats about 200 and this year a 1620s comedy by Philip Massinger, ''A New Way to Pay Old Debts,'' is to be played there from June 13; and from August 3, ''The Dillen.'' This is set in Victorian Stratford, and is a look at the lives of the very poor of that time and place.
What else? If Bath happened to be on your schedule (it should be, as it's a delectable city), then a visit to the newly restored Theatre Royal is a must. Not much of its summer program is known at present, but it will be ''very open, '' according to a spokesman. In fact, this is not one of the regional theaters that originates its own productions. It is used to house top touring productions (the National Theatre, for example, comes here) and promises a rich menu this summer. The National Theatre production of John Gay's ''The Beggar's Opera,'' starring Paul Jones, runs June 21 to 25. The director is Richard Eyre, recently responsible for the National Theatre's highly acclaimed ''Guys and Dolls.'' ''Cowardice'' by Sean Mathias, starring Janet Suzman and Ian McKellen, follows July 11 to 16, and a successful London piece, based on New Yorker Helene Hanff's love affair with a London bookshop, ''84, Charing Cross Road,'' will be seen July 18 to July 23.
John Gay's sequel, ''Polly,'' is also doing the rounds this summer. From July to August the Cambridge Theatre Company is taking it to the University of Warwick Arts Centre, Coventry, June 28 to July 2; The Playhouse, Oxford, July 4 to 9; Theatr Clwyd, Mold, in North Wales, July 11 to 16; the Cambridge Arts Theatre, July 19 to 30; and the Theatre Royal, Brighton, August 1 to 6.
Cambridge itself boasts an Arts Festival in July, and at the Arts Theatre, apart from ''Polly,''there will be a play by Agatha Christie, ''Cards on the Table'' (June 20 to 25); ''Habeas Corpus,'' by Alan Bennett (July 4 to 9); and ''Sweet Charity,'' the musical based on a play by Neil Simon (July 11 to 16). At Oxford, the playhouse is visited by a varied array of things - puppets and poetry; modern dance; a university review (''The Footlights'' from Cambridge - July 11 to 16); and a production of ''Hamlet'' (June 13 to 18). 84, Charing Cross Road will also be seen there June 6 to June 11.
Oxford also has a summer festival of open-air theater, ''Summer in Oxford, ''surely attractive to overseas tourists: ''Medieval Players'' in the cloister at Magdalen College; mime, dance, drama; amateur Shakespeare in Trinity College gardens; and more.
Heading up the east coast, doughty summer theatergoers might well aim for the seaside resort of Scarborough. The attraction? Well, it could be escape from an overdose of drama (and traveling) in the form of attractive beaches dotted with rock pools, or it could be the ''theater in the round,'' which provides its own delightful and funny forms of theatrical escapism. It's a converted concert room above a library. And it has its own resident playwright - a determined provincial if ever there was one - who also happens to be its artistic director. His name is Alan Ayckbourn. ''Intimate Exchange'' is his latest hit, a play that after the first five seconds can lead, plotwise, in a total of 16 directions. They've had to resort to a diagram to explain it to the bemused public. Some of these 16 variations may well be seeable in Scarborough this summer - only two actors make up the cast and they have performed it 117 times since it opened in June last year.
Ayckbourn wrote in 1974 that he enjoyed the restrictions of the little ''theater in the round'' at Scarborough, because he preferred small casts and scenic limitations. He also got on very well with the artistic director, and he liked the changing audience of holidaymakers in the summer. When I asked if he still feels the same way in 1983 even after his name has become internationally known and his plays have been performed all over the place, this man of many words gave me a succinct reply, saying, ''I get happier by the day in Scarborough.'' What more need be said?
The small Century Theatre in Keswick, on the shores of Derwentwater in the Lake District, is just one company to perform Ayckbourn plays (among other things); his play ''Time and Time Again'' opens June 21 this year. This summer's program, which continues until October 1, also includes ''The Phantom of the Opera'' (opening June 21) and ''The Importance of Being Earnest'' (opening June 29) just the sort of easy theater to attract visitors to this glorious area, as a good way of ending a day of hiking or driving.
Although the Century is now stationary, its productions are not. They are performed all over, including one of the prettiest and smallest theaters I have seen, the beautiful Georgian Theatre in Richmond, in Yorkshire. Anyone interested in theater history should not miss a visit to this tiny, 18th-century delight. Devoted volunteers conduct tours.
At the northernmost point of this meandering itinerary there is the Pitlochry Festival Theatre in a lovely setting in the hills exactly in the middle of Scotland. The brochure says ''the ambiance'' is unique - leisurely playgoing, excellent meal before the show, a greeting from the kilted manager, a view of river and mountains from the foyer. Guest directors and guest designers are responsible this year for five productions - all fairly ''safe'' fare. They are ''The Admirable Crichton,''(May 7 to September 3) ''Night Must Fall,''(May 11 to September 24) ''Rookery Nook,''(May 14 to September 24) ''Twelfth Night,'' (June 10 to September 21) and ''Translations,'' by Brian Friel (July 8 to September 22 ).
If ''summer'' extends to the last week of August and the first two of September, then it's not far from Pitlochry to Edinburgh and that city's annual international festival of the arts, August 21 to September 10. Here, if anywhere , can be encountered ''dangerous'' drama as well as delightful, experimental as well as predictable. For instance, this year the Haifa Municipal Theater is bringing a play that caused controversy in Israel, ''The Soul of a Jew'' (first week). The Glasgow Citizens' Theater is bringing two new productions (in repertory for three weeks) - a new translation of the play ''Der Rosenkavalier, '' and a four-hour epic, a prophecy of the rise of Hitler, described to me as ''Viennese satire - strong stuff.'' It is called ''The Last Days of Mankind,'' by Karl Kraus.
And - as always - there will be a flood of other theatrical events, national and international, old and new, amateur and professional, outrageous and lovable , indoors and out. . . . Practical details
Phone numbers of theaters mentioned above in order of appearance: Chichester Festival Theatre 0243-781312; Yvonne Arnaud Theatre 0483-60191; Royal Shakespeare Theatre 0789-295623; Theatre Royal, Bath; 0225-65065; University of Warwick Arts Centre 0203-417417; The Playhouse, Oxford 0865-247133; Theatre Clwyd 0352-55114; Arts Theatre, Cambridge, 0223-352000; Theatre Royal, Brighton, 0273-28488; ''Summer in Oxford'' Festival; 0865-242060; Theatre in the Round, Scarborough, 0723-70541; Pitlochry Festival Theatre 0796-2680; Edinburgh Festival 031-226-4001.