Despite repeated boasts that he would breach Britain's ban, Idi Amin was the big man who wasn't there today as Commonwealth chiefs began their biannual meeting here.

The conspicuously empty chair of the Ugandan president overshadowed the gaithering of 33 governments - Britain and her former colonies.

But the cunning Amin continues to spread an air of mystery over his whereabouts and a menacing air over the 250 Britons still in Uganda.

Kampala Radio said none of the Britons, mostly missionaries and a scattering of businessmen, could now leave the country. Their "state," the radio said ominously, will be decided "one President Amin returns from the Commonwealth Conference in London."

Amin has threatened the Britons before but officials here take every threat seriously. "There is no knowing what he will do," one official said. "The man is dangerous. He is feeling humiliated."

Even those professionally required to find out confessed today they have no hard information on Amin's whereabouts. He is said to be sending out decoys to cloak his whereabouts. One trick he used yesterday was to send planes aloft with hints that he was aboard. The confusion was deepended when a former Aer Lingus pilot hoaxed Dublin airport and led authorities to think Amin was landing there.

The best guess here is that Amin has left Kampala, but only to go to Liby and his ally, Col. Muammar Q. Addafi. There, Amin presumably can nurse his injured pride and plot more mischief with a compatible, oil-rich nation.

Tonight Prime Minister James Callaghan urged reporters to "keep cool" over the flurry of reports about Amin's whereabouts and intentions. "Don't let Amin think he's getting under our skin," callaghan urged. Where then was the Uganda chief? "In Kampala," Callaghan said flately.

His foreign minister, David Owen, also tried to minimize the affair. But Owen acknowledged that he was concerned about the safety of the Britons in Uganda. "It's always an anxiety of mine," he said.

The order banning Britons from leaving Uganda was issued by Vice President Gen. Mustafa Adrisi, who is in charge in Amin's absence. Adrisi also warned the British government not to attempt any military rescue operation and accused London of sending a reconnaissance plane over Uganda this morning.

A similar ban was imposed on about 240 American nationals in Uganda five months ago after President Carter said reports of mass arrests and killings in Uganda "disgusted the entire civilized world." That ban lasted four days.

Britain has not publicly banned Amin's presence here but Callaghan has made clear that the Ugandan, viewed with loathing as a mass murderer, is not welcome.

Another vacant chair, that of the Seychelle islands, also drew attention today. President James R. Mancham, the Seychelles swinger, was overthrown Sunday while here for the conference.

The new regime headed by Albert Rene asked the islands' high commissioner (ambassador) here to take Mancham's place at the commonwealth meeting. But the high commissioner, Jean Georges Rassool, announced in a letter to the Times of London today that he is loyal to Mancham.

The coup, Rassool wrote darkly, "was carried out with the help of a Marxist foreign power." Mancham himself had blamed the Soviet Union for his downfall.

According to Commonwealth officials, more than ideology link Mancham and Rassool. The high commissioner is said to arrange and join in Mancham's partying and nightclubbing in London.

Karen Rene, former wife of the Seychelles' new president, also shed some light on the coup in an interview. No living in a London suburb, she began by describing her ex-husband's attendance at Communist Party meetings, but she ended by indicating that the real source of the split was turtle soup rather than Marx.

Rene, she said, was upset by a Mancham business venture that involved slaughtering the islands' turtles. To preserve the animals, Rene called for a five-year ban on their killing and thus cost Mancham a tidy sum.

Commonwealth leaders today decided that no delegation from the Seychelles would be seated, pending a hearing for Mancham.

The leaders who are here will spend much of their time denouncing black oppression by white rulers in Rhodesia and South Africa. The big question is whether they will also denounce Amin's oppression of his own people.

Kenneth Kaunda, president of Zambia, who has been on the verge of war with Amin several times, has been denouncing the Ugandan dictator from every microphone put in front of him. But other third world states are reluctant to apply a single standard to white and black regimes. They claim that to do so is to interfere in the internal affairs of a sovereign, liberated state.

The newest Commonwealth country represented here, Papua-New Guinea, did blast both Uganda and the South Africans. In the end, the expectation is that the conference will announce a "consensus" deploring internal oppression without naming Uganda.