Mrs. Thatcher sets up her own advisory team

British Prime Minister Margaret Thatcher has created a new advisory team to help her develop foreign and defense policies at 10 Downing St.

And for the first time in history, the Foreign Office and Defense Ministry will not be a British leader's primary source of guidance on policy toward the rest of the world.

Mrs. Thatcher's choice for the key post of foreign policy adviser is Sir Anthony Parsons, until July Britain's ambassador to the United Nations. The appointment of Sir Anthony, widely acknowledged to be one of Britain's most brilliant diplomats, has been the cause of serious tension between the prime minister and her foreign secretary, Francis Pym.

Until now the foreign secretary has been a British prime minister's chief source of foreign policy advice. In future Sir Anthony, working closely with Roger Jackling, a senior official seconded from the Defense Ministry, will funnel advice to Mrs. Thatcher inside her private office in Downing Street.

Mr. Pym at one point opposed the change and is still said to resent the new arrangements.

But since she assumed office in 1979, Mrs. Thatcher has had her doubts about the advice she was getting from the Foreign Office. The problem came to a head with the Falklands crisis, when she was given only a few days warning of the Argentine decision to invade and seize the islands.

At that time Lord Carrington was foreign secretary, but he resigned in the uproar. Relations between Mrs. Thatcher and Mr. Pym, Lord Carrington's replacement, have been described as frosty.

Sir Anthony Parsons has been called in to keep the prime minister fully up-to-date on all aspects of foreign affairs.

He is an Arabist and spent much of his early career in Middle East countries. He was ambassador to Tehran at the time of the Shah's overthrow and freely acknowledges that he erred in failing to predict the Iranian ruler's fall.

He is a straight talker and during the Falklands crisis once asked Mrs. Thatcher not to interrupt one of his briefings. Friends say he has voted Labour at least twice.

Sir Anthony is also an expert in English literature. He taught himself Anglo-Saxon to be able to read Beowulf and Chaucer. He says his new post will largely consist of liaison work between 10 Downing St. and the Foreign Office.

But his appointment, together with Jackling's, may mean that Mrs. Thatcher is preparing to create a full-scale prime minister's office similar to the advisory staff of an American president.

The prime minister's officials deny this, but Mrs. Thatcher already has a full-time economic adviser, Prof. Alan Walters, and a home policy adviser, Ferdinand Mount.

During argument over the Parsons appointment, Mr. Pym warned Mrs. Thatcher that the creation of a prime minister's department could not be reconciled with the British Constitution.

Sir Anthony has a reputation for inspiring deep trust. At the UN he managed to deliver all the Security Council votes needed to secure a mandatory resolution condemning the Argentine invasion. Whitehall admirers say it was the most effective and stylish diplomatic maneuver ever carried out by Britain in the world forum.

In some ways Mrs. Thatcher's new adviser is a colorful figure. In New York he was famous for his careless style of dress and was said to be engaged in a ''crooked tie competition'' with Britain's ambassador in Washington, Sir Nicholas Henderson.