"My feelings I with difficulty smother," sang Sir Oliver Wright, British ambassador to the United States, "When diplomatic duty's to be done/ Ah, take one consideration with another,/ An ambassador's lot is not a happy one."

The British ambassador was, as his formal title proclaims, both extraordinary and plenipotentiary, last night in the Terrace Theater. His performance was easily the highlight of the Washington Opera "Follies," against competition that included Metropolitan Opera baritone Robert Merrill, D'Oyly Carte star John Reed, Eddie Albert, Geraldine Fitzgerald, ballerina Martine van Hamel, the entire cast of "L'Italiana in Algeri" and, by no means least, his own wife Lady Marjory Wright. All the performers donated their services for this benefit program.

Sir Oliver, who participated in last year's "Follies" only as a member of an ambassadors' trio, may have been lured into his solo role when he saw how much fun Lady Marjory was having last year. He introduced her last night, "for the second time at the Washington Opera 'Follies,' and at even greater expense than the last time -- the one and only Lady Marjory Wright."

She sang Cole Porter's "The Physician": "He said that my cirrhosis/ Was his favorite diagnosis,/ But he never said he loved me."

Sir Oliver's number, modeled on "A Policeman's Lot is Not a Happy One" from "The Pirates of Penzance," was rewritten by Sir Oliver and retitled "The Ambassador's Song" from "The Envoys of Penzance." At the dinner after the show, he said that it had been written "in a long Anglo-American tradition of friendship and mutual mockery." Martin Feinstein, general director of the Washington Opera, confided that the ambassador "held a competition among his staff to write the words, but finally wrote them himself." They were "not for attribution," the ambassador said -- too late.

"When the Senate's not engaged in its employment/ Of improving David Stockman's budget plans," he sang, "Bob Dole's capacity for innocent enjoyment /Is just as great as any honest man's."

Later he turned to other members of the Reagan administration:

"When Cap ain't sounding off about his Star Wars/ He loves to read a Churchill tome or two,/ When Mac Baldrige ain't protecting U.S. Steel bores/ He'll gladly rope a steer for me or you/ When Don and Jim ain't swapping with each other,/ They love to lie a-basking in the sun/ Ah, take one confusing Post leak with another,/ An ambassador's lot is not a happy one."

But he found some consolation in his life, exiled here in the colonies:

"When the president's chopping wood on his vacation,/ And George's having 18 holes instead,/ Your ambassador for a little relaxation/ gets to watch the Redskins on TV in bed/ He'll write a letter to his dear old mother/ And do the things that normal people done,/ Go to one delicious opera after another, /Then your envoy's lot is really rather fun."

He did not mention Michael Deaver, deputy chief of staff at the White House, perhaps because Deaver was also on the program. "You may have heard of him," said Feinstein. "He did something at the White House and recently resigned. There has been some speculation on what he will do next. What he does now may or may not be a clue." Deaver played a set of variations on "Memory" from "Cats" for two pianos and orchestra, in partnership with pianist George Cort. His admirers may hope that he does not have to do it for a living, although he seemed to enjoy doing it for fun, and so did the audience. The title of his selection may or may not have had a thematic connection with recent events in his life.

Asked about his job plans, Deaver said, "I don't know yet; a pianist's life wouldn't be a bad one. I love playing the piano." Pressed further, he talked about "Memory": "I think it's one of the greatest songs that have come out of Broadway in the last 10 years; it's going to be another 'Stardust.' "

The purely artistic part of the program included a good mixture of the popular and the classical, the sublime and the ridiculous. Among the sublime parts of the evening were Martine van Hamel dancing "The Dying Swan"; Mimi Lerner, star of "L'Italiana," singing two songs by Jacques Brel; Robert Merrill singing Figaro's first aria from "The Barber of Seville"; Eddie Albert in a heartbreaking performance of "September Song"; and a series of Spanish dances for two pianos played by Misha and Cipa Dichter. Geraldine Fitzgerald sang a segment from her one-woman show "Streetsongs," focusing on what she called "la-la songs -- songs for which we don't know all the words." These included the theme from "Alfie," "One Fine Day" from "Madame Butterfly" and "The Poor People of Paris." Dina Merrill (no relation to Robert, though they call one another "cousin") sang a Kurt Weill song, "One Touch of Venus," that bordered on an X rating.

Sir Oliver was not the only one who sang Gilbert and Sullivan, nor technically the most impressive. There was considerably more musicianship, if not quite so much celebrity value, in selections from "The Gondoliers" and "The Mikado" as sung by John Reed and Joanna Levy, a team that was perfect in style and impressive vocally.

Feinstein, normally a timid soul, took the podium during the final curtain calls, conducting "Stars and Stripes Forever" while all three of the evening's conductors took their bows. "I'm a specialist," he said at the dinner. "I do the 'Hallelujah' Chorus and the bow music at the 'Follies.' I leave other conductors free to do everything else."

All of Rossini's most popular comic operas have at some climactic point a number in which five, six or seven people sing, beautifully and at great length, about how confusing the situation is. Last night's gala concluded with the edition of this number contained in "L'Italiana," the septet that ends its first act with the entire cast singing in ensemble. It was an impressive demonstration of what the Washington Opera is all about, and how well it performs in its best moments.

Vaudeville is not dead, as this evening demonstrated. But now it costs $250 a ticket with dinner at the Watergate thrown in.