The penchant for secrecy here, a device to protect Britain's governing elites, has suddenly turned into a snake biting its own tail. Two events last week, both involving secrecy, have seriously undermined confidence in the honesty of government across the board.

The event that received the most attention was a not overly searching government inquiry into a 12-year breach of trade sanctions against Rhodesia. The report of the inquiry by lawyer Thomas Bingham into the continuing supply of oil by British (and American) firms to the white regime produced no surprises. But it amounted to an acknowledgment by the government that its foremost officials had collaborated with the great oil companies to provide Ian Smith with lifegiving oil.

At the same time, the trial of two journalists and a social worker for allegedly violating the Official Secrets Act blew up in the government's face.It was rocked by the disclosure that the jury foreman, a former professional intelligence commando, had been pressing his fellow jurors for a conviction before any evidence had been heard.

Even more staggering, the judge in the case wanted to prosecute still another reporter who had revealed the foreman's occupation rather than prosecute overeager commando.

Assent is the very essence of a democracy. Taxes can't be collected, laws won't be obeyed unless assent is freely given. Even so stable a government as Britain's is not likely to govern long if its essential integrity is questioned.

A cartoon in Monday's Daily Mail makes the point. It shows gleeful Ford company executives sitting around a table while their managing director says, "If we break the pay limit, the government has threatened to impose the most stringent sanctions - like they did in Rhodesia. Isn't that great?"

Prime Minister James Callaghan's government is trying to fight inflation by winning assent to a 5 percent limit on wage increases, and Ford has become a test case. The cartoonist is shrewdly suggesting that neither Ford nor its union need take Callaghan too seriously now.

The consequences of Britain's prolonged masquerade over Rhodesia - pretending to be indignant over the supply of oil from others, even sending the Navy to blockade a port where no oil entered, and all the time ensuring the bulk of supplies from Shell and British Petroleum - have yet to be measured, at home and abroad. An indignant President Kenneth Kaunda of Zambia has called former premier Harold Wilson a "liar." That brought Callaghan rushing to see Kaunda over the weekend, and according to informed diplomats, Callaghan had to assuage Kaunda's wrath with promises of sizable aid.

But it is not only Wilson whose integrity has been questioned. As The Economist magazine observes, his private secretary, Michael Palliser, received secret accounts of how the oil companies were delivering the goods.As Sir Michael, Palliser is now head of the Foreign Office and responsible for Britain's assurance to the United Nations in 1976 that no British oil was going to Rhodesia. Palliser, in turn, "relied" on an assurance from Sir Frank Macfadzean, then the head of Shell and now chairman of British Airways. Both assurances were fraudulent.

The "secrets" trial aroused disquet even before it began early this month. Two journalist, Duncan Campbell and Crispin Aubrey, face 14 years for interviewing John Berry, a social worker who seven years ago served in Britain's radio-monitoring intelligence net. The trio were seized before a line was published. That aroused fears in conservative quarters that the government was imposing prior restraint, attempting to squelch publication.

Moreover, the three are charged under two highly questionable provisions of the law. One is aimed at punishing espionage, but the government has said that none of the three was spying. The other is so wide-ranging a ban against the mere giving and receiving of unpublished information that Callaghan's home minister had earlier promised it would not be used again.

But it is the latest revelations over the foreman, a long-serving member of the dreaded Special Air Services, that has really caused the deepest trouble. His presence on the jury and his extraordinary lobbying there go right to the heart of British justice.

This affair raises the question whether the government deliberately rigged the "secrets" case jury. The prosecution could, and did, examine the background of every prospective juror. But the defense was unable to do that: Under the British system it could not even put questions to prospective jurors; it could only challenge them on the basis of their looks.

Britain's stability over the centuries has rested on rulers capable of adapting to changed circumstances, and change has come about gradually. That has meant that feudal elements coexist with microelectronics, a House of Lords with Tribune Socialists. But it also means that aristocratic qualities have survived in governing circles. One such characteristic is the belief that the business of government is too precious, too complex to share with the governed.

That attitude has been bolstered by a wide range of legal devices to choke off inquiry: harsh libel laws, the doctrine of judicial contempt, the Official Secrets Act.

Today, the cult of secrecy has never been under so much fire. Papers are challenging attempts to muffle disclosures in "open" courts in a way that would have been unthinkable a few years ago. The somnolent House of Commons is pressing for committees that can really ask questions. There are calls for a Bill of Rights to arm Britons with protected free speech.

Bingham's inquiry into Rhodesia is an unusually revealing statement of facts - no matter how muffled its conclusions. But the British press, from all sides, is insisting that it is not enough, that a public tribunal must now fix responsibility for the game played on British citizens and the world.

The government has scheduled the "secrets" trial to start all over again, with a new jury, on Oct. 3. Between now and then, it may have second thoughts.

Whatever decision is taken, the past week has impressed upon many thoughtful Britons that open government is not merely a special-interest plaything of the press. It is coming to be seen as a necessity for the sound functioning of government itself.