It was a bit cheeky of these American lawyer chaps, coming to London and setting up court to ask questions about a uranium cartel.

That wasn't the worst of it. These Yankee lawyers brought their own judge, Robert R. Merhige, a fellow from somewhere called "the Eastern District of Virginia."

This federal judge, as they called him, held forth, not in proper British Court of Law, but over at the U.S. embassy on Grosvenor Square. And the Americans put their questions to some very proper British businessmen.

Well, one thing led to another and Sir Mark Turner and Lord Shackleton of Burley (he heads the Honors Committee, you know, the one that selects knights and all that), and five other executives of Rio Tinto-Zinc declined to answer.

Actually, as the Americans put it so inelegantly. Sir Mark and Lord Shackleton and the others "took the Fifth." The Fifth seems to be an amendment in their Constitution which protects citizens against self-incrimination.

Now imagine - British citizens compelled invoke the U.S. Constitution to avoid testifying before an American judge who is holding court in London. A bit daunting.

But that's not the end of it. Judge Merhige accepted the American lawyers started talking about something called the Omnibus Crime control and Safe Streets Act. lt seems this law allows the U.S. Attorney General to arrange criminal immunity for persons who plead the Fifth - promising not to prosecute them in any U.S. court, then compelling them to testify.

So the Yankees went back to their nation's capital and talked to their Attorney General, Grifin Bell, and yesterday he issued the proper papers for a grant of immunity.

Now if Judge Merhige consents, the whole blooming lot will head back to London - where Sir Mark and the others from RTZ will be compelled to "sing" about uranium price-fixing and other unpleasant affairs.

This international courtroom battle may sound like something from a British force, but it is actually serious American legal drama - with billion dollar implications. The American lawyers represent Westinghouse Electric Corp., which defaulted two years ago on uranium-supply contracts and is non being sued for damages by 20 or so public-utility companies before Merhige in Richmond.

But Westinghouse claims in defense that it was squeezed out of the world uranium market and that the prices were escalated fantastically by an international cartel of uranium producers from five countries, including RTZ in London and the Canadian Subsidiary of Gulf Oil Corp., among others.

Westinghouse field its own lawsuit in Chicago against 29 uranium-producing firms, including RTZ. Many of the foreign-based producers have refused to respond to this suit, contending that their foreign activities are no business of the U.S. courts.

But Samuel A. Haunbbold, Chicago attorney for Westinghouse, wanted to take depositions from RTZ executives because that global minerals corporation was regarded as one of the prime movers in the once-secret cartel. Haubbold resorted to an international legal device called "letters rogatory," dating from an age when kings used to ask other kings for favors.

The British Court of Appeal held in May that it was in the interest of British law for Westinghouse to question the RTZ officials about the cartel - perhaps to discover whether Common market laws against price-rigging might also have been violated.

ln Britain only corporations, not individuals can be held liable for anti-trust violations.

Last month, after Merhige held his hearing at the embassy in London he was given a certificate attesting that he was the first U.S. judge to hold court there normally such matters on depositions are handled by consular officials.

This might be an intercontinental joke among lawyers, save for two matters.

For one thing the stock of RTZ has falled drastically on the London exchange folowing the unpleasant publicity about the cartel. The local newspaper made some rather unkind references to the Fifth Amendment being a favorite of America's Malla gangsters.

More important, a U.S. grand jury in Washington is looking into the cartel too. The immunity grant means that none of the seven RTZ executive can prosecuted, but it does not prevent the Justice Department from using their testimony toward indicting other play ers in the cartel, either companies or individuals. This includes RTZ a British corporation which has subsidiaries in America.