National security adviser John M. Poindexter assured a top British diplomat on May 28 that the United States stood firmly by its policy of not making deals for hostages, according to U.S. and European sources. On that same day, Vice Adm. Poindexter's predecessor, Robert C. McFarlane, landed in Tehran in a plane loaded with U.S. arms, expecting that all U.S. hostages in Lebanon were about to be released.

The British were particularly concerned that Washington adhere to its professed policy of refusing to make concessions to hostage-takers, informed sources said. Prime Minister Margaret Thatcher had not only refused to bargain for release of two Britons held by pro-Libyan forces in Lebanon, she had ignored the danger to the hostages and, in a show of solidarity with President Reagan, allowed U.S. warplanes based in Britain to be used in the April 15 bombing of Tripoli.

In retaliation for that raid, the two Britons and one American were executed by their captors. Thatcher has been criticized for her position.

Poindexter met last May 28 with Sir Antony A. Acland, then the permanent undersecretary of the British Foreign Office and now the British ambassador to the United States. Poindexter gave Acland no hint that McFarlane was then carrying U.S. arms to Tehran but said the administration was considering various options for getting the hostages out, according to these sources.

A spokesman for Poindexter said last night that the admiral could not "pin down" the May 28 conversation, but added that Poindexter had often discussed Iran, the U.S. arms embargo and "the nonnegotiability" of hostages with British officials.

British questions may have been provoked by suspicions raised after Israeli officials and Saudi Arabian tycoon Adnan Khashoggi asked a well-known British businessman to help deliver arms to Iran.

The British businessman, Roland (Tiny) W. Rowland, was asked whether he could participate in a secret operation described to him as backed by the White House. Rowland then contacted the State Department to confirm that the operation had American backing and was told by top State Department officials that they knew nothing about it.

These pieces in the incomplete jigsaw puzzle of the White House-directed secret Iran operation came to light yesterday:

"The two other undisclosed passengers on the plane that flew former national security adviser McFarlane and Lt. Col. Oliver North of the National Security Council staff to Tehran May 28 were retired major general Richard V. Secord and George Cave, a retired Middle East specialist for the Central Intelligence Agency who had served as station chief in Iran during the time of the shah and in Saudi Arabia.

"Southern Air Transport, which has been tied to a plane involved in secret arms shipments to the U.S.-backed antigovernment rebels, or contras, in Nicaragua, also provided planes for the White House Iranian operation, according to one administration source. McFarlane was aboard a Southern Air-provided plane, which also carried arms, when he flew to Tehran, the source said.

"CIA Director William J. Casey told legislators Friday in closed session that the Iranian arms operation was run in a fashion similar to the contra aid program during the period when the CIA was barred from direct participation in the Nicaraguan operations.

"Casey indicated the CIA was not one of the major players" in the Iranian arms operation, said one legislator who attended the briefing. Another participant said he described the agency as "mainly in a support role," although Casey and his CIA general counsel had drafted the presidential "finding" that set much of the secret plan in motion, according to other sources.

Information about the attempt by Israeli officials and Khashoggi to enlist the help of Rowland adds a dimension to the unfolding story of U.S. efforts to ship arms to Iran via various intermediaries in hopes of establishing contacts with the Tehran leadership and gaining the freedom of Americans held in Lebanon by pro-Iranian extremists.

The Israelis, including David Kimche, director general of the Israeli foreign ministry, had been involved in arranging two shipments of U.S. arms from Israel in September 1985, when the first of three American hostages to be freed, the Rev. Benjamin Weir, was released.

Rowland's Lonhro company was involved in sanctions-busting operations for the former white government in Rhodesia and has provided financial and other assistance to the U.S.-backed Angolan rebel leader, Jonas Savimbi. Rowland and Kimche, a former Mossad agent, are close friends.

In late April or early May, Kimche, Khashoggi and Amiram Nir, an Israeli counterterrorism expert who was Israel's liaison to the secret Iran operation run out of the White House, met with Rowland and asked if he would be interested in air shipments to Iran that would involve grain, medical supplies and arms. The first two items apparently were a cover for the third.

Rowland, suspicious that the United States could approve arms shipments to Iran, wanted this verified by the Americans. When he contacted State Department officials, the idea of such an operation was greeted with surprise, sources said. Rowland was advised not to become involved.

He took the advice and refused to participate, the sources said.

Subsequently, State Department officials who had no knowledge of the secret White House program went to Poindexter to inquire. Poindexter stonewalled the officials but left them with the impression that the White House had something under way, according to knowledgeable sources.

In early April, Poindexter told McFarlane that U.S. efforts to make contact with Tehran were bearing fruit and later that month said McFarlane might be asked to go to Iran, according to informed sources. McFarlane went to Israel May 27 and boarded a Boeing 707 there for Tehran.

According to one source, the charter aircraft, also carrying weapons and spare parts for the Iranians, was arranged for by Southern Air Transport, a company that was CIA-owned from 1960 to 1973. Since 1983, Southern Air, which owns at least three 707s, has received $ 90 million in business from the U.S. Air Force.

Accompanying McFarlane, according to several sources, was Secord, a former deputy assistant secretary of defense in charge of military assistance who has been identified as working in the past with North in the U.S. contra aid program. Southern Air has been linked to aircraft used in that program.

Former CIA Middle East expert Cave served as translator for the group because he is fluent in Farsi, the main Iranian language, according to two sources.

The group spent four days talking to low-level Iranian officials before giving up in frustration in efforts to contact top Tehran officials.

McFarlane, who had expected to see all the American hostages in Lebanon released as a result of his trip, accused the Iranians of "extreme bad faith."

A second shipment of U.S. arms, which was supposed to be sent after the hostages were out and McFarlane departed, was not sent, according to informed sources.