BRIZE NORTON AIR BASE, ENGLAND, DEC. 7 -- Prime Minister Margaret Thatcher said after talks here this morning with Soviet leader Mikhail Gorbachev that a "common sense" compromise on space-based defenses could provide a "way ahead" for forging new U.S.-Soviet arms control agreements.

Thatcher discussed her compromise proposal, which she first made to Gorbachev and President Reagan early this year, at a two-hour meeting with Gorbachev during a refueling stop on his way to Washington.

The plan formed the centerpiece of what she later described as a "thoroughly valuable, and of course stimulating" visit between the two leaders, who last met when Thatcher went to Moscow last spring. The two leaders are said to enjoy a close if combative relationship, and they seemed glad to see each other as Gorbachev bounded down the steps of his Aeroflot Ilyushin 62 jet into the chilly, sunny British morning with what appeared to be genuine enthusiasm.

Thatcher aides insisted that the meeting, which was set three weeks ago, was not intended to upstage the Reagan-Gorbachev summit, but rather to give the Soviet leader an "opportunity to herald in London the message he's going to deliver in Washington."

Gorbachev, the first Soviet leader in 31 years to pay an official visit to Britain, said in reference to the treaty to eliminate intermediate-range nuclear forces (INF) that "the road to the agreement which we have come very close to signing . . . was not an easy one. But we have covered this road together, the Soviet Union, the United States, Great Britain, our allies and your partners."

"We expect our visit to Washington, our conversations with the administration of the United States, with other interlocutors, with the American public, will help us move forward on the road of restructuring of international relations to a better and deeper cooperation and mutual understanding," Gorbachev said. "That's what we aim for."

As she did during her own Moscow trip, Thatcher effusively praised Gorbachev and his campaign for internal reforms in the Soviet Union, saying, "He's a bold, determined and courageous leader, and I hope that he succeeds with his task." Calling for world backing for his efforts, she said, "You can't be urging human rights and then not support glasnost and perestroika" -- Gorbachev's policies aiming for more open political debate and a restructured Soviet economy.

Thatcher reportedly believes much of her power derives not only from her position as the West's longest-serving current head of government, but also through her position as Reagan's closest ally and confidant in Europe. Although she did not mention Reagan, her message at a subsequent news conference appeared addressed toward Washington when she noted that "you have to be very, very well prepared" for superpower summit meetings. "During the course of the talks," she said, "new ideas may come up. That often happens . . . but you don't make any commitment. You go away and work them out, work them up. I'm not expecting any shocks."

Gorbachev and Reagan on Tuesday will sign an INF agreement eliminating those missiles deployed primarily in Europe. However, both men have said that progress toward a separate accord involving a proposed 50 percent cut in long-range strategic missiles will be at the top of their agenda. Those negotiations have been blocked by disagreement over space-based missile defense programs, specifically the U.S. Strategic Defense Initiative (SDI), designed to counter the long-range weapons.

Thatcher said her proposed compromise is a way around the logjam. In the news conference, she described it as "quite a significant step which makes further arrangements . . . possible," such as a strategic agreement.

Her plan calls for both Washington and Moscow to pledge adherence to the 1972 Antiballistic Missile (ABM) treaty for a fixed period, probably 10 years, while observing its provision not to deploy space-based systems.

She also believes that greater mutual predictability and reassurance would be aided by the exchange of regular "timetables" in which both sides would describe their upcoming space defense research plans.

"I understand how destabilizing uncertainty could be," she told reporters. "We would like to see a reaffirmation, an undertaking, that there would be negotiations before deployment . . . together with indications from both sides of their intentions." Summing up her position, Thatcher said: "I stand on common sense."

As it has been described by Thatcher and her aides, the plan is more in line with Soviet views on ABM interpretation than those of the Reagan administration. Although Gorbachev did not commit himself today, British officials indicated a high level of Soviet interest and said they now believe the ball is in the U.S. court.

The United States has officially rejected Soviet calls for testing and deployment restraints on SDI. The administration argues that testing is permitted under its interpretation of the ABM treaty and that deployment, while not planned in the immediate future, could be accommodated by withdrawal from the ABM treaty, as is allowed within the provisions of the accord itself.

The Soviets first proposed a 10-year moratorium on scrapping the ABM treaty during the Reagan-Gorbachev summit last year in Reykjavik. Gorbachev demanded a commitment to the ABM treaty and restrictions on SDI before discussing strategic cuts.

In recent days, however, both sides have made conciliatory gestures. In his interview last week with NBC News, Gorbachev acknowledged for the first time that the Soviets are conducting their own strategic defense research, which he described as on a par with the American SDI program and liable to the same restrictions he proposes for Washington. Meanwhile, U.S. officials have hinted they may be closer to accepting a long-term commitment to the ABM treaty.

"I have the impression it is not only being considered, but has been talked about quite a bit," Thatcher said today of her proposal.

At her press conference, Thatcher sought to make clear her recognition of the fact that Britain does not sit at the superpower table. "I am a staunch and loyal ally of the United States within NATO, and in no way a go-between," she said of her talks with Gorbachev. "That was the whole basis of the discussion."

But she made clear that her role is one of substantial influence on both superpowers. Commenting on her talks with Gorbachev, she described the atmosphere as "very, very good indeed. . . . It usually is when Mr. Gorbachev and I get together. It was an animated debate. He's a powerful person, and I don't think I'm anything other than that, too."

Although Gorbachev was received at this Royal Air Force base 70 miles west of London with all the honors due a visiting head of state, ceremony was kept to a minimum. Thatcher, wearing a dark woolen suit, greeted him at the foot of a ramp rolled up to the Soviet plane, with Foreign Secretary Geoffrey Howe and Leonid Zamyatin, the Soviet ambassador to Britain, at her side.

The two leaders dispensed with a review of the assembled 80-man military honor guard, and a program of three numbers by the Royal Air Force band was shortened to just the Soviet national anthem. The official party quickly climbed in a motorcade of black limousines and drove to the base officers' mess, where Gorbachev and Thatcher held an hour of one-on-one talks while Howe met with Soviet Foreign Minister Eduard Shevardnadze. Meanwhile, Raisa Gorbachev was whisked away to tour a nearby primary school.

All met for a hurried lunch of salmon and Chablis. Gorbachev and Thatcher walked outside, eight minutes behind Gorbachev's scheduled take-off time for Washington, to give departure statements.

Thatcher said the INF treaty would be "a source of extra security and joy" during the Christmas season, "not only for the people of our two countries and of Europe and the United States but also for the world as a whole.

"And it's a promise of more progress to come," she added.

The brief visit gave both leaders a chance to reaffirm their close relationship that began when Gorbachev first visited Britain in December 1984, shortly before he ascended to the Soviet leadership. But it also provided both leaders with an opportunity to make points to domestic, regional and international constituencies.

For Gorbachev, the stop here was a chance to press home his insistence that the Soviet Union is above all a European country that shares the security and economic concerns of West Europe and is eager to play a constructive regional role.

For Thatcher, Gorbachev's presence was an opportunity to send a message to U.S. congressional opponents of the INF accord, disabusing them of the notion that Western Europe is against it. She and other West European leaders had initially expressed doubts about the wisdom of the treaty, but they now say they embrace it wholeheartedly.

"We want to tell the Senate that we don't need to be rescued from it by saviors on white horses," said one British arms control official. "We are going into this with our eyes open."

At the same time, Thatcher wanted to impress upon Gorbachev that, while Western Europe approves of the accord, it wants no further nuclear weapons cuts in Europe until the massive Soviet superiority in conventional weapons is addressed and balance is achieved.

The Soviets have indicated their willingness to talk about cuts in the shortest-range category of nuclear missiles in Europe -- those with a range of 300 miles or less that are not covered by the INF treaty. But much of Western Europe, with the exception of West Germany, agrees with Thatcher that some nuclear deterrence still is needed until the conventional imbalance is addressed.

"I think Mr. Gorbachev understood what I was saying," she said. "The next steps following INF and 50 percent reductions in strategic weapons" must be cuts in conventional and chemical weapons. Until these concerns are addressed, she said, "there is no scope for further {nuclear} reductions in Europe."

Thatcher said somewhat pointedly, in response to a question, that she did not expect any "surprises" at the Washington summit. The reference was to Western Europe's dismay over the near-agreement in Reykjavik between Gorbachev and Reagan over the elimination of all nuclear weapons in one sweep. "The point about nuclear weapons," she said, "is that they deter, and have so far prevented, war in Europe."

British officials have said since the Reykjavik summit that they believe Reagan and U.S. negotiators were unprepared for Gorbachev's sweeping proposals and were saved from a disastrous agreement only by Gorbachev's insistence on SDI restrictions that Reagan refused.

Thatcher traveled to Washington soon after that meeting for talks at Camp David in which, according to her closest aides, she "put Reagan back on track" by expressing her own and Europe's concerns and lowering his sights on a "world without nuclear weapons."

There was some speculation before today's visit that Gorbachev would press Thatcher for cuts in Britain's own independent strategic nuclear deterrence, the submarine-based Polaris missiles. But Thatcher indicated that this did not form a substantial part of their conversation.

"I made clear that I regard our nuclear deterrent as our last resort, an irreducible minimum," she said. "Until negotiations have gone a lot further than 50 percent, there is no question of bringing ours in. . . . If it goes a lot further, we have said that we would consider it."

She said she also touched on the subject of human rights, although Howe indicated that the issue had been principally stressed in his discussions with Shevardnadze.

While she said she pressed for more human rights progress, Thatcher indicated that some positive steps have been made by Gorbachev. She said he should be encouraged to proceed, rather than publicly chastised for occasional lapses such as yesterday's KGB attacks on Jewish refuseniks demonstrating in Moscow.

"We were on the very broad issues," she said. Yesterday's incident, she said, "was tragic. But one must keep broad progress and the direction of progress in mind, and not sacrifice the possibility of increased, accelerated" reforms.

"The important thing," she said, "is that progress on human rights goes on, and that it be stepped up." Saying she "wished Mr. Gorbachev well" in his domestic efforts, Thatcher said his policies of glasnost and perestroika were major factors in improving relations between Britain and the Soviet Union.

"Relations are good," she said. "I think they are better than they have ever been before."