FIRST A TOAST to the cow, the British gentleman in elegant formal dress announced grandly, raising a glass of chilled champagne towards a disinterested bovine placidly grazing just yards away from his picnic basket, spread on the lawn of the English country estate. His three fellow picnickers--two women in evening gowns and another man in white waistcoat and black tie--took turns saluting their own favorite elements of the gala evening: the neat wicker hamper packed with biscuits and brie, strips of smoked salmon and goose liver pa te', the fresh Sussex air surrounding the stately manor house and the magnificence of Mozart whose opera "Idomeneo" was the central reason for this special outing to the opening of the 1983 season of the Glyndebourne festival opera.

What Wimbledon is to tennis and Ascot is to horse racing, Glyndebourne is to opera--a uniquely British summer spectacular, steeped in tradition, where the spectators and trappings are as delightful as the performance itself. It is an experience as different from fried chicken at Wolf Trap as a Miller break is from tea time.

Glyndebourne's charm lies in its successful blend of seeming incongruities: a black-tie picnic, a world-class opera house in a country town more than 50 miles from London, a pastoral--yet grand--setting.

The main attraction is excellent opera, performed in a gem-like theater, accompanied by the London Philharmonic orchestra and directed by such greats as the National Theater of Great Britain's Sir Peter Hall and the Royal Shakespeare Company's Trevor Nunn.

"Glyndebourne is one of the few places in the operatic world where the director's needs are treated as seriously as the conductor's," says Sir Peter , who is currently in Germany directing Wagner's Ring Cycle at the theater festival of Bayreuth, Wagner's home theater. "Glyndebourne's work has been founded on the need for close collaboration between conductor and director. That is the only way that real operatic drama expressed by music is made. Glyndebourne's contribution to British opera and British theater has therefore always been central."

Two principles form "the heart of Glyndebourne," says festival chairman George Christie. "Glyndebourne is the total experience of music and drama, and it does not promote the star system." This philosophy goes back to the start of Glyndebourne, says Christie, who is the son of founders John Christie and opera singer Audrey Mildmay. They launched the first festival in 1934 with productions of two Mozart operas--the Marriage of Figaro and Cosi fan tutte--both perfectly suited to the 330-seat theater (later enlarged to seat 800) that Christie built adjacent to the Sussex manor house.

The project some considered a rich man's folly won early success, largely because two opera greats left Germany for Glyndebourne because they couldn't work under the Hitler regime--conductor Fritz Busch of Dresden and producer Carl Ebert from Berlin. Among the many other exceptional talents associated with Glyndebourne's past: Rudolf Bing, who served as general manager before going to the Metropolitan Opera in New York, and artists Luciano Pavarotti, Dame Joan Sutherland, Regine Crespin and Sir Geraint Evans who sang at the festival early in their careers.

While the opera is the evening's star, the event itself is the real show with all the staff and audience the players.

The experience begins at Victoria Station--unless you plan to drive or be driven, Glyndebourne style, by a uniformed chauffeur behind the wheel of a Rolls. Since our limo was in the shop (British humor) we traveled by train to the opening of Glyndebourne's 1983 season.

Dressed in our finery, we felt somewhat awkward among the Mohawk-shaved punk rockers at Victoria Station until we saw a woman in fur and man in velvet walking toward Track 15. (At that point my husband and I felt undressed, despite my knee-length silk dress and his simple jacket and tie.) We knew we were literally on the right track--the 2:53 p.m. to Lewes--when a glance through the train window revealed an elderly, formally-dressed couple with a straw hamper at their feet. They chatted calmly, with the casual air of people who dress up and take the train every day, paying no attention to us or to the gaggle of schoolgirls in blue blazers and broad-brimmed hats skipping down the aisle to the rear seats.

Within half an hour we had slipped outside suburban London into the soft green countryside. At Lewes two large buses awaited what turned out to be several dozen elegant opera-goers and drove us 10 minutes through the village to the Christie estate: a spectacular structure of ivy-covered stone.

Although we arrived just in time for tea, the line at the Mildmay Tea Room dampened our desire for scones and cream. Instead, we stopped at the restaurant desk to order our meal--served during the 75-minute interval--then followed the rest of the crowd out to the lawn. Picnicking at Glyndebourne began simply because there weren't enough restaurant seats to go around. But many of those who rushed out to claim favorite picnic spots on the croquet lawn or in the urn garden prefer dining outoors.

"We've been doing this since 1958," said Michael Wheaton of Surrey, seated on the lawn with his wife, Ann, and guests Hildegard and Ralph Renouf of Jersey. "It's a perfect start to the summer season."

We stopped at the bar for the house drink, a Pimm's Cup--a fruity, sweet concoction of wine and lemonade--before joining in the informal pre-opera promenade along the lawns. The sound of champagne corks popping provided perfect accompaniment for the occasional melancholy lowing of Friesian cows--whose black and white coloring fit the formal occasion--grazing in a nearby field. Faintly, as if in the distance, we heard the trill of musical scales from choral members limbering up their voices.

When the bell sounded around 5:15, the picnickers wrapped their goodies in their blankets and left everything on the lawns, confident that nothing would be disturbed--unless, of course, by rain, one of Glyndebourne's few hazards.

Inside the opera house, even the ushers were elegant, dressed in dinner jackets accented by gold medallions on red ribbons. The production of "Idomeneo" marked the opera debut of director Trevor Nunn (of "Nicholas Nickleby" fame, and who earlier this month won a Tony award for directing the musical "Cats"). One of Mozart's least-produced, most difficult works, it is the story of a king of ancient Crete who inadvertently dooms his son by asking the God Neptune's help in surviving a storm.

Exquisitely costumed and brilliantly staged against a minimalist decor of Japanese-like screens and symbolic lighting (a shadow falling across a harsh white glare signified the monster in Act II), the production was--to quote the London Times--"a model performance of one of the supreme works of art." Despite the glorious voices and music, however, the audience remained reserved, holding applause until the end of the act.

The dinner interval ran quite smoothly: our Beaujolais had been opened to breathe, and our stuffed avocado and watercress soup appetizers were waiting on our candlelit table in the Nether Wallop restaurant. Although we had been limited to cold menu choices since we had ordered when we arrived and not 48 hours in advance as recommended, the meal was quite nice. While hardly a gourmet's delight, the roast beef was nicely rare and the poached salmon finely textured, with salad, bread and strawberries and cream the cost was 32.29 pounds sterling ( 1 is worth approximately $1.60 at the current exchange rate).

The meal-mellowed audience was considerably more effusive during Act III, lavishing enthusiastic applause on the cast with an especially appreciative ovation for American Carol Vaness as Electra.

"The audience is always more relaxed after the dinner interval," says the 30-year-old singer who made her debut last summer. "Glyndebourne is a little frightening at first since the audience seems practically on top of you if you're used to a theater like the Met that holds around 4,000."

As a result, she says, "everything is done slowly and carefully with incredible attention to detail. Here they usually spend a month and a week rehearsing. In other places I've spent as little as a week."

After the opera some of those who didn't have to catch the train lingered for a last drink, while the rest of us climbed aboard the buses that whisked us to the waiting train. The return trip went quickly in the darkness and deposited us at Victoria Station--like Cinderella--before the stroke of midnight.