Last night Raymond Leppard and Richard Stilwell were the special attractions in a Bach program at Wolf Trap, where this week Leppard will lead all of the Brandenburg concertos, two cantatas, and on Sunday - night an all-Handel program. For Leppard it is onlyone of a series of engagements through which he hopes to establish himself as a major conductor in this country, both in opera and the symphoic field.

The series also onpening night of the Metropolitan Opera's 1978-79 season. With Stilwell in the title role, the British conductor will be in charge of the Met's first production of Benjamin Britten's opera, "Billy Budd."

Leppard, long known as on of the world's eminent authorities in Baroque music, a graduate of and for 10 years lecturer at Cambridge, and one of England's busiest musicians, has renounced his British citisenship and is applying for citisenship in this country.

In town this week for his Wolf Trap concerts, which are his first appearances as a conductor in the Washington area, Leppard stated his reasons for deciding to move to the United States. "I do not like a socialist way of life," he said, "and that is the life has become in England."

If Leppards's name is extremely familiar to record collectors, it is almost completely thanks to his remarkable work as conductor, performer on the harpsichord, and editor of the baroque operas of Monteverdi and Cavalli. While he says very frankly that "Recording are to me something of the past, they are done and I never listen to them, because what is exciting to me is the present and the future," it is recordings of the Bach Brandenburgs, suites, and harpsichord concertos, Handel's "Messiah," and concerti grossi, overtures and the big orchestral pieces like the Royal Fireworks Music and the Water Music that have made Leppard widely known and admired in this country.

When Wolf trap presents Cavalli's comic opera "L'Egisto" in August, it will be in Raymond Leppard's edition. When the Glyndebourne Festival in southern England gave a spectacular production of Monteverdi's "The Coronation of Poppaea" some years ago, it was heard and recorded.

Today Leppard says,"I do not think of myself as a baroque specialist. That was certainly very helpful earlier in my carrer when I was getting started. But I hope to conduct a broad repertoire with orchestras in this country. They have asked me if I would include some English music, and of course I want to. I am thinking of the First Symphony by Walton, and then I wonder what you would think of the symphonies of Bax."

There are seven symphonies by the late Sir Arnold Bax who held the title of Master of the King's Musick at the time of his death in 1953. Neglected for a time, they have recently been enjoying revivals by several conductors of whom Leppard has been one of the more active.

Leppard has never confined himself exclusively to baroque music. Last summer he made his first appearances at the Santa Fe Opera conducting virgil Thompson's "The Mother of Us All," which he subsequently recorded. In St. Louis this past season he conducted Rossini's comic opera, "Le Comte Ory," and his conducting at Glyndebourne included the world premiere of Nicholas Maw's "The Rising of the Moon."

Nevertheless when he goes back to England for the limited period of time he will now be allowed to work there under his new status, he will conduct recordings of two great Handel works: the oratorio "Samson," and the opera "Ariodante," in both of which Janet Baker will star.

There is still another side to Raymond Leppard. He has composed the music for three major films, including "Alfred the Great." "It was 'Alfred,'" he said in an interview, "that bought me my house in London, the one I have now sold." And he said "sold" without the slightest visible trace regret.