At a crucial meeting at 10 Downing Street last weekend with Prime Minister Margaret Thatcher, her foreign secretary, Lord Carrington, began lecturing the visiting president of Zambia, Kenneth Kaunda, on the British position at the Rhodesian settlement conference here.

"Now I see what the Patriotic Front has been talking about," said Kaunda, referring to complaints by the Zambian-backed Patriotic Front guerrilla leaders, Joshua Nkomo and Robert Mugabe, who had said that Carrington was running the conference like an arrogant British schoolmaster.

Before Carrington could react, Thatcher intervened.

"Fasten your seat belt, Peter," she told Carrington.

"And observe the no smoking sign," muttered a Kaunda aide, impressed.

In the end, that meeting led to one last round of concessions by both the British and the Patriotic Front that helped produce agreement on the British plan for transition to majority rule in Zimbabwe-Rhodesia.

The meeting also illustrated how the British have made so much progress toward a peace settlement in the southern Africa civil war and how Thatcher's often zealous Conservative government generally works.

Carrington's stern manner and tough bargaining in the Rhodesia talks have helped force the two warring sides to accept difficult compromises put forward by the British.

His stance also has enabled Thatcher, when necessary, to use him as a foil in her dealings with intermediaries like Kaunda, representing African and British commonwealth nations with influence over the Patriotic Front. Thatcher, who originally had to be persuaded by Carringon to bargain with the guerrillas in an all-party peace conference, now appears as a flexible statesman alongside a hectoring Carrington.

A peace agreement, which now appears attainable by the end of the month, would give Thatcher a major triumph in her first foray into international diplomacy. She would be able to flaunt this achievement on her visit next month to Washington, where she felt she had not been taken seriously during talks with the Carter administration two years ago when she was the British opposition leader.

The United States, Britain's partner in the previous unsuccessful attempt to achieve a Rhodesian settlement, has played almost no role in the negotiations being run here by Thatcher and Carrington. American diplomats have merely been kept up on developments by the British. They have also provided timely assurances of U.S. financialaid to help with resettlement of exiled guerrillas in Zimbabwe-Rhodesia, and compensation for rich farm land that a future black majority government may expropriate from white farmers.

Britain's negotiating tactics have relied on asserting British authority as the colonial power in Rhodesia, Thatcher's unswerving sense of purpose once she believes she is on the right course, and Carrington's step-by-step brinksmanship in demanding difficult concessions from both Prime Minister Abel Muzorewa and the Patriotic Front.

Referring to differences of interpretation that arose today over the transition agreement, Mugabe said it was possible the British were using "a Kissinger approach" of telling one side one thing and the other something else.

Some cynics feel, in fact, that the British are putting off the hardest problems until the transition stage is reached in Rhodesia, where the precarious package could fall apart, unleashing an escalated war, with Britain caught in the middle running the show.

The British tactics are typical of Thatcher's determination to pursue policies to the limit with bold, almost reckless determination, no matter how high the stakes. It is embodied in her survival-of-the-fittest approach to Britain's ailing economy, taking one radical step after another, despite the risk of making the situation even worse before it can get better.

Thatcher is now launching an attempt to end a decade of sectarian strife in Northern Ireland in the same way. She seeks to force Protestant and Catholic leaders to agree to share power in a limited form of home rule by threatening to impose her own solution if they don't agree.

"it's the gag and swallow approach to negotiations," one diplomatic observer said of the British tactics in the Rhodesian peace talks. "The British decide what they think is the best compromise and then force the two sides to accept it after they've had a chance to complain a lot."

In this observer's view, Carrington's stern conduct of the conference has made it easier for the two warring sides to compromise because "they can save face with their followers back home by saying the British were such bastards, it was the best deal they could get."

As it happens, the British have had to gag and swallow some themselves. They have gone much further than expected at the beginning of the conference in involving themselves inside Rhodesia during the transition to legal independence and in safeguarding the interests of the Patriotic Front guerrillas, whom Thatcher previously had always referred to as Communist-backed terrorists.

The British have now committed themselves to sending hundreds of their civil servants, police officers and troops to Rhodesia to help run new elections and avert trouble between the opposing sides during the election campaign.

Previously Thatcher has said she did not envisage using British troops in Rhodesia.

The British have also promised to help move the guerrillas and their followers back into Rhodesia from exile in neighboring nations and to feed and house their forces during the transition. British officials acknowledge that all this will cost Britain many millions of dollars.

"These negotiations actually have been going on at several levels," said one observer. He pointed out that each British concession, made at the urging of intermediaries like Zambia's Kaunda, had to be negotiated between Thatcher and her diplomats.

Other sources indicated that similar shadow negotiations have been going on inside the Patriotic Front, with Nkomo, backed by Kaunda, much more ready than Mugabe to make concessions to the British at each crucial stage. In addition both guerrilla leaders have been subjected to pressure from their African supporters.

The British did not at first welcome outside influences in the negotiations and Carrington at one point scolded Commonwealth Secretary General Sonny Ramphal for interfering.

Carrington did not contact the Commonwealth secretariat here before approaching members like Australia, New Zealand and Fiji about contributing to a small Commonwealth military force to monitor a cease-fire in Zimbabwe-Rhodesia.