President Francois Mitterrand and British Prime Minister Margaret Thatcher met Wednesday on a rocky islet off the coast of France to discuss their differing visions of a united Europe in 1992 and relations with the Soviet Union.

The socialist president and the conservative prime minister were to hold informal afternoon talks and planned to tour the medieval fortified abbey at Mont St. Michel, a chunk of granite along the northwestern coast that becomes an island only at high tide.The leaders arrived by presidential helicopter just after 1:30 p.m., and went to the century-old La Mere Poulard, a restaurant famous for the flaming omelets of the same name.

At the restaurant, Mitterrand and Thatcher watched as two chefs prepared their lunch, whipping the eggs up with a samba-like rhythm in a copper skillet.

The business lunch was expected to focus on such issues as relations with the Soviet Union and this week's European Community summit meeting on the Greek island of Rhodes.

The meeting is Mitterrand's first with another Western European leader since his return Saturday from the Soviet Union, a subject of interest to Thatcher as she prepares for Soviet leader Mikhail Gorbachev's visit to London in December.

Mitterrand had seven hours of talks with Gorbachev last week during a two-day trip to Moscow and Baikonur, where he watched the launch of a joint French-Soviet space flight carrying a French cosmonaut to the Soviet station Mir.

The talks in Moscow produced agreements for increased cooperation in space and set the stage for a series of political, economic and commercial exchanges next year between France and the Soviet Union, highlighted by a visit by Gorbachev to Paris in the spring.

Thatcher, known for her tough-mindedness in dealing with the Soviets, is scheduled to meet Gorbachev Dec. 12 to 14.

Those meetings have more potential for contention than Mitterrand's cordial talks with Gorbachev because of Thatcher's complaints about the slow pace of improvements on human rights issues in the Soviet Union.

She has urged her Western allies to boycott an international human rights conference in Moscow in 1991 unless Gorbachev complies with the Helsinki accord on human rights.