BRIZE NORTON, England — Prime Minister Margaret Thatcher met early Monday for two hours of what she called "animated debate" with Mikhail S. Gorbachev, urging the Soviet leader to reach additional arms control accords with the West and offering a compromise formula to ease Moscow's concerns about the U.S. Strategic Defense Initiative.
"The important thing is that the new spirit of arms control continues," she told newsmen after the visit.
She praised Gorbachev's campaign for wide-ranging domestic reforms as steps toward liberalization within the Soviet Union that would help ease political tensions.
"He's a bold, determined and courageous leader and I hope that he succeeds in his colossal task. . . ," she said.
The 56-year-old Soviet leader, looking relaxed and confident, made only a brief statement to reporters before departing for Washington and his summit with President Reagan.
Call for Cooperation
Gorbachev said he hoped the Washington summit would "help us move forward on the road of the restructuring of international relations to a better and deeper cooperation and mutual understanding."
Gorbachev made his remarks shortly after concluding his talks with Thatcher, held at the officer's mess of a large Royal Air Force base near this village, 60 miles west of London.
Gorbachev and his wife, Raisa, plus an entourage that required four giant Ilyushin 62 aircraft to carry, stopped for 2 1/2 hours here on their way from Moscow to Washington.
In addition to meeting the need to refuel the Soviet aircraft, Gorbachev's brief stop here was seen as an expression of his wish to stress the importance that Moscow attaches to West European opinion and as an indication of the unusual personal rapport that he has developed with Thatcher, a leader of completely opposing ideology but remarkably similar personality.
During their talks, Thatcher revived an idea that she first mentioned early this year and raised again during her visit to Moscow in March--proposing that both the United States and the Soviet Union pledge to observe at least seven more years of the terms of the 1972 Anti-Ballistic Missile (ABM) Treaty and notify each other of intended research programs in the space defense field.
"We need the confidence as well as balance for security," she said.
Six Months' Notice
The treaty now allows either signatory to withdraw on six-months notice. Gorbachev wants a 10-year commitment to the pact, which strictly limits the deployment of anti-missile systems such as envisioned under the SDI program, known as "Star Wars." The extent to which it bars testing of such a system in space is an area of disagreement between the Reagan Administration, some in the U.S. Congress and the Soviets.
Thatcher said Gorbachev had given no indication during their talks of a willingness to pick up her proposal of a seven-year accord on the ABM treaty but argued that his statement during a television interview last week that Soviet scientists are also conducting an SDI program might facilitate such an idea.
"I think that, of itself, was a significant step, a quite significant step, which makes further arrangements of the kind I've been talking about possible," Thatcher said.
She insisted, however, that her proposal did not place her in the role of an intermediary between Gorbachev and Reagan.
"I am not a go-between," she said. "I'm quite an important part of the NATO Alliance and I'm a very reliable ally. No one has any doubt where I stand, but I believe that other people have the right to defend their way of life and I understand the need for them to feel secure in their defense."
'An Historic Treaty'
Reflecting the position of many other West European leaders, Thatcher praised the accord that the two leaders are scheduled to sign today, to eliminate all ground-launched intermediate-range nuclear weapons, calling it "an historic treaty."
She repeatedly stressed the need for the United States and the Soviet Union to move quickly on agreements to reduce by half their arsenals of intercontinental ballistic missiles and to make balanced reductions of chemical and conventional arms in Europe, where the Soviet Bloc has a large numerical superiority.
On the Soviet troop presence in Afghanistan, Thatcher said Gorbachev gave no indication if Moscow would meet a timetable that she proposed for withdrawal of Soviet troops by the end of next year.
On internal Soviet reforms, Gorbachev reportedly indicated a determination "to drive on," but in the area of human rights, there were signs of toughness within his delegation.
Talks on Rights Spurned
According to one British source, during discussions on the subject between Foreign Minister Geoffrey Howe and his Soviet counterpart, Eduard A. Shevardnadze, who conducted separate, parallel talks during the stopover, Politburo member Alexander N. Yakovlev stated flatly, "Human rights is not an issue that should be discussed between countries."
Later, the British prime minister phoned Reagan for 20 minutes to give him an account of her talks with Gorbachev.
As Thatcher and Gorbachev discussed global issues, Gorbachev's wife, dressed in a silver and white fur coat, visited an elementary school near the base to see a Christmas play. She apparently left a strong impression with some students.
One pupil, unsure of her exact role, stated: "I know she's important, because she runs Russia."