British Ambassador Peter Jay has come up with a unique way to end his diplomatic assignment here. He will sail June 29 for England via a 2,630 mile trans-Atlantic sailboat race.

The voyage winds up Jay's 23 months here as envoy, a career abruptly terminated earlier this month when his father-in-law, former Labor Prime Minister James Callaghan, was defeated at the polls by Conservative party leader Margaret Thatcher.

Jay, 42, arrived in Washington in July 1977, on the crest of controversy in London over the peremptory firing of his predecessor, Sir Peter Ramsbotham, by then-Foreign Minister David Owen.

Critical of Ramsbotham's style and "fuddy-duddy" image, the Callaghan government was outraged by reports of his "lavish" parties and film star/socialite guest lists, in view of Britain's faltering economy.

It was at a Ramsbotham bicentennial reception for Queen Elizabeth that he and his wife arranged a blind date between actress Elizabeth Taylor and Virginia politician John Warner.

The choice of Jay aroused cries of "nepotism" from Conservatives, although some political observers saw the appointment as an attempt by Callaghan to establish closer ties between his government and the then-fledgling Carter administration.

Married to the former Margaret Ann Callaghan, the youthful Jay, former economics editor of The Times of London, quickly settled into a diplomatic life style in Washington that emphasized a low public profile. Unlike Ramsbotham's few of Jay's embassy parties ever were reported in the press. His White House connections, on the other hand, were impressive on several levels, including that of his children and President Carter's daughter, Amy, who became friends through a special after-school reading course at George Washington University.

Jay submitted his resignation shortly after Thatcher's victory. Embassy officials confirmed yesterday that Jay will sail back to England starting June 29, in the Irish Cruising Club's Golden Jubilee race, a voyage between Marblehead, Mass., and Crosshaven outside Cork, in Ireland, expected to take between 15 and 18 days.

Jay, described as "more of a cruising man than a racing one," co-owns the 12-year-old, British-built sailing sloop Norvantes with John Hooks, owner of an engineering firm in Croydon, England. Hooks sailed the 47-foot vessel this spring to the British Virgin Islands where Jay and his family boarded it during the Easter holidays. It has since been taken to Bermuda.

The Norvantes is one of 15 boats entered in the race, to be run jointly by the Eastern Yacht Club and the Cruising Club of America, one of the sponsors of the biennial Newport to Bermuda race.

Described as "about as wide open as a race can get - you can radio for any reason, unlike the Bermuda race," John C. Kiley Jr., a cruising club officials said the route is not considered particularly dangerous, "not at all."

Kiley said the ambassador's name "came out of the blue" sometime in March. Until then, he had never heard of Peter Jay before. "It's a pretty good way to go home," Kiley said, adding that "he won't be able to get all his stuff in the boat though."

An experienced sailor, who has been hoisting sail for 20 years, Jay once participated in the "Round Britain" race, one of the most grueling tests of seamanship England has to offer.

An embassy official said Jay and his wife, a former BBC-TV producer, were "not quite sure" what they will do after the race (Margaret Jay will not be among her husband's crew).

"They might settle here," the spokesman said. "Whatever he does, he's going to write a book first. CAPTION: Picture, Peter Jay, by UPI