"I would like to thank fate, not only for the weather," said outgoing British Ambassador Peter Jay, his champagne glass raised, as he looked across the dinner tables clustered on the green of Wolf Trap Farm Park, awash in brilliant evening sun, "but ambassador coincide with the first night of the Royal Ballet." (See review on page B3).

Jay spent his last night in official-dom with a few senators and congressmen, Elliott Richardson, (a former ambassador to Britain), and the founder of the Royal Ballet, Dame Ninette deValois, among others, who sat down to a small black-tie outdoor dinner before attending the opening night of the Royal Ballet at Wolf Trap.

The ambassador did not dwell on the post he was leaving, but talked with great anticipation of leaving today for a three-week boating race from Marblehead, Mass., to Portsmouth, England.

"You dream about it during the winter," he said with a smile, about the race which starts on Friday. He will sail his yacht "Norvantes." "That thought keeps you going. For me, this will be a total release."

But he looks with even more relish to the fall, when he settles back down in Washington with his family, probably renting a house in the Embassy Row area not far from where he lived as ambassador. "I'm going to write a book," he said, "which is something I've been agitating about for the past five years."

The subject: "In the area where politics and economics meet," said Jay, a former economics editor and columnist for The Times of London. "It will be about why our economics don't work the way we think they should, and what we should do about that."

Jay said he will examine the economies of developed western democracies. "It's a very ambitious task," he said, "We need to go back and start in a different direction. I expect some of the ideas will be quite radical - in the strict sense of that word.

"I'm tremendously looking forward to writing this book," he added. "It will be the one important thing I'll do."

Some guests were still expressing their sympathy at his leaving his post.

"I'm horrified to hear you're leaving so soon," said Joan Leslie to Jay.

"Well, it all comes to an end, doesn't it?" said Jay with a smile.

"I came to know and like the Jays in my all-too-short tour of the U.K.," said Richardson, adding he also had warm memories of the incoming ambassador, Nicholas Henderson, a longtime diplomat, called out of retirement for the job by new Prime Minister Margaret Thatcher.

"I know Nick well," said Richardson, grinning. "I won two wonderful clarets from him. I was betting on Ashe against Connors in Wimbledon in '75. Henderson is a wonderful connoisseur. I'm sure I got paid quite well."

Jay, who has a long-standing reputation as an intellectual, dazzled some guests with his rapidly delivered toasts.After thanking Kay Shouse, founder of Wolf Trap, for the dinner and for Wolf Trap, which Jay called "one of the glories of Washington," he concluded. "An ambassador strives away trying to promote good relationships between his country and the one he is in. But one night of the Royal Ballet here is worth 100 nights of 100 ambassadors working away in all parts of the country."

The Jay family will spend August at their house in County Cork, Ireland. In the fall Margaret Jay will do news and public affairs production for National Public Radio here during their coverage of the SALT debates on the floor of the Senate, she said.

She may also do "a little producing and a little reporting" for Channel 13 in New York. "But that's not firmed up," said Mrs. Jay, a former BBC-TV producer in England.

There are some things she will not regret about leaving the embassy. "It will be nice to have my own house with my own front door. I'd like to live in a house where if I shout at my children they can hear me." CAPTION: Picture, Ambassador and Mrs. Jay, at left, with Mr. and Mrs. Miles Copeland; by Lucian Perkins - The Washington Post