Britain's open-air theaters; FROM SHAKESPEARE TO SHERIDAN

A tour of Britain's open-air theaters might strike some people as an antic caper, a commendable but rash pastime sure to provoke the elements, like storing winter clothes in April. It may come as a surprise, therefore, to learn that 1982 marks the 50th anniversary of two notable al fresco theaters, the Minack in Porthcurno, Cornwall, and the Open Air Theater, Regent's Park, London. Open-air productions are also given at Polesden Lacey, Surrey, and at Ludlow Castle, Shropshire.

In 1932, when Rowena Cade of Porthcurno began shifting the rocks on the hill by her clifftop house to make a theater, skeptics abounded. But by summer a terrace theater had been sculpted beneath the precipice, and a group of Cornish players presented ''The Tempest'' high above Mounts Bay.

Each year thereafter Miss Cade, assisted by Billy Rawlings and C. T. Angove, made improvements. The theater now seats 600. Etched on the lower seat backs are the names of famous productions (''Twelfth Night,'' 1933; ''Henry IV, Pt. 1,'' 1959), sermons in stone facing the actors.

''Plays with a strong, simple story come off best. Those not cluttered up with too many props, which can be a nuisance if the weather turns bad,'' Miss Cade told an interviewer for The Times of London in 1968. The reporter praised her ''cliff-hanging drama'' but declared, rather peevishly, that the Minack was ''no Glyndebourne'' and that the audience, laden with blankets, cushions, and food, dressed in trousers, had the appearance of ''refugees or members of a civil defense exercise.'' But that same year Charlton Heston cabled from America for tickets to ''The Beggar's Opera'' and said it was his chief reason for going to Britain.

It is safe to say that most people come not as refugees but as ardent pilgrims and connoisseurs of drama. If tempests come, everyone endures stoically; the cast then applauds the audience (as they did after one rainy matinee of ''Amphitryon 38'' I attended). Blankets, steaming hot drinks, and snacks are obtainable halfway down.

Theatrical companies from all over Britain vie to perform here. This year's productions include ''The Tempest,'' ''Lark Rise at

Candleford,'' ''Tristan of Cornwall,'' and ''Dark of the Moon.'' The season runs from the last week in June to the first week in September; tickets are $3. 40. To reach the theater, come by taxi or car from Penzance, nine miles away. Buses are infrequent.

The cliffs are as striking as those at Tintagel, the theater as majestic as the ancient Roman one at Taormina. Plays mounted in this high aerie, in Prospero's words ''. . .'twixt the green sea and the azured vault,'' can scarcely fail to please.

If ''The Tempest'' is the perfect play for the Minack Theater, the Open Air Theater in Regent's Park virtually demands ''A Midsummer Night's Dream.'' It is in the tradition of the Italian garden theater, with background trees and shrubs complemented by such architectural features as trellises and steps. An illuminated Renaissance palace, a marvelous fake, looms at the rear, fostering the illusion that one is a cherished guest on a private estate.

Fortunately ''A Midsummer Night's Dream'' is frequently given, and a better setting can scarcely be imagined than the hint of Theseus's lofty palace, the idyllic bower for Titania, and the tumbling foliage for Puck, Peaseblossom, and Moth.

The theater was founded in 1932 by Sydney Carroll and Robert Akins, though performances had been given there from 1901 to 1904 by Sir Ben Greet & Co. Since 1962 the New Shakespeare Company, under the direction of Sir David Conville, has run the theater.

''The English summer is a much balmier and sunnier happening than the jokesters make out,'' Sir David says. Only 7 percent of performances have been canceled over the past 20 years; a free ticket is then offered for any other performance. He urges visitors to come early and take a leisurely walk through Queen Mary's Rose Garden. Sandwiches, barbecue, and other refreshments are obtainable in the foyer, or you may picnic.

The theater seats 1,200 and is rarely sold out. The 1982 season runs from late May to late August and features ''A Midsummer Night's Dream,'' ''The Taming of the Shrew,'' and a double bill of two Shaw plays. Tickets range from $3.80 to

It is fitting that Polesden Lacey, Surrey, should be the site of an outdoor theater, for Richard Sheridan, author of ''The School for Scandal'' and ''The Rivals,'' once lived in a house on this site. The present mansion was adapted in 1906 from an 1824 Regency villa. A National Trust property, it has been kept as it was in the days when Mrs. Ronald Greville, a noted Edwardian hostess, entertained royalty. It is set in nearly 1,000 acres of park, woodland, and farmland.

The theater has a grass-banked stage against a backdrop of fine trees, with a distant view of the house. Productions have been given here since the 1951 Festival of Britain. R. L. Snow, general manager, says the more expensive seats sell out quickly; ''even the vagaries of the English weather do not deter such keen support.''

Since 1953 Elsie Green has been the tireless producer. ''Her dedication,'' says Snow, ''has meant that nothing second-rate would do.'' She is now assisted by Kay Dabney.

Visitors may tour the house and grounds and dine in the cafeteria before the play. This summer ''The Tempest'' will be given June 30-July 3 and ''The Importance of Being Earnest'' July 7-10. From June 23-26 there will be an Edwardian fete. Tickets range from $2.85 to $6.65. Postal bookings are accepted as of April 1. Write the Polesden Lacey Open Air Theater, Great Bookham, Dorking , Surrey, England RH5 6BD. The theater is 25 miles from London; trains run from Victoria.

Ludlow Castle, in Ludlow, Shropshire, near the Welsh border, also has special claim to theatrical eminence. In 1634 Milton's masque ''Comus'' was first performed here. It was written for John, Earl of Bridgewater, Viscount Brackley, on the occasion of his becoming lord president of Wales.

Ludlow itself is well worth the journey. It was a 13th-century wool town, and many timbered buildings survive. One of the most splendid is the 13th-century Feathers Hotel, with a richly decorated interior and carved exterior; it has been an inn since 1609.

''Richard III'' will be presented June 29-July 10. Other events are also scheduled. Ludlow may be reached by car or by train from Shrewsbury. Tickets range from $3 to $13. Write E. Leonard Mundy, Secretary, Castle House, Ludlow, Shropshire SY8 1AX.

Britain's open-air theaters span theatrical history from the Athenian theater of Dionysius to the Renaissance court and garden theaters. The visitor, bewitched by sea-battered rocks, multi-textured foliage, and the celestial dome above, may well ask himself: Why, after all, submit to a walled and artificial playhouse? For I have been in Arcadia.