Britain is sending its senior professional diplomat as its new ambassador to Washington, Prime Minister Margaret Thatcher's government announced today.

Antony Acland, currently serving as permanent undersecretary of state and chief of the diplomatic service, will replace outgoing Ambassador Oliver Wright in early September. Wright, whom Thatcher called out of retirement to take over the Washington embassy 4 1/2 years ago, is expected to leave in July and will not take another diplomatic post.

The office of permanent undersecretary is the highest in the British foreign service and traditionally is followed by retirement. According to Foreign Office officials, the Washington embassy -- Britain's largest and most important -- is the only posting that can be considered a step up for someone in Acland's position.

"It is the one job that, if anybody asked me throughout my career, I unquestionably would have said I'd like to go be ambassador to Washington," Acland said today in a briefing for American correspondents here.

His appointment, which he said already has been approved by the Reagan administration, comes at a time when Thatcher is under sharp domestic questioning for the current close alignment between U.S. and British policy in a number of areas.

Last month, Britain became the first -- and remains the only -- Western European country to sign a formal agreement to participate in the administration's Strategic Defense Initiative, despite bilaterally negotiated terms that appeared substantially less than what the Thatcher government originally sought.

At the same time, Thatcher authorized Britain's pullout from UNESCO, following the U.S. example and recommendation, over significant objection from within her own government as well as from a broad spectrum of opposition political opinion.

Neither action appears to have provoked noticeable public reaction or controversy. But they have become a focus of concern for some within the governing Conservative Party, where a traditional tug of war exists between those who believe Britain's best interests lie in closer ties with Western Europe rather than the United States.

The most recent reflection of these often opposing views concerns rival American and European bids for a takeover of Westland, Britain's only helicopter manufacturer. Thatcher's defense secretary, Michael Heseltine, has strongly backed a proposal by a European consortium on grounds that British security is best served by less dependence on the U.S. defense establishment. But Thatcher appears to favor a bid by Sikorsky, the American helicopter company, and reportedly has rebuked her defense chief for interference.

In his session with American reporters today, Acland said he did not think cooperation between Britain and the United States "can be too close." But, he said, "I rather deplore the tendency always to suggest" that Britain has to take sides between Washington and Western Europe.

"There shouldn't be competition between the two," he said. "We are geographically part of Europe." But "I don't think that means ties with the United States should necessarily be weakened," he said.

Acland spoke of the "intensely close relationship between the prime minister of the day" and the U.S. "president of the day -- particularly between Thatcher and Reagan."

He said that relations between Britain and the Reagan administration had been greatly improved by continuing indications that "the U.S. administration does value dialogue" with its allies, and said that such consultation was "particularly intense at the moment on security issues and East-West relations . . . which this year will probably be the most important issue."

Educated at Eton and Oxford, Acland entered the foreign service as an Arabic-speaking specialist, and held postings in Dubai and Kuwait before becoming head of the Arabian department in the Foreign Office. He also has served as a British envoy to the United Nations in New York and Geneva.

In 1972, Acland became principal private secretary to the foreign secretary. Three years later, he became British ambassador to Luxembourg and, in 1977, ambassador to Spain. He returned to London as deputy undersecretary of state in 1980, and was appointed to his current job in 1982.

Acland, 55, is a widower, with three grown children.