London — Americans wondering why Western Europe so actively opposes parts of the Reagan strategy toward the Soviet Union might take a long look at the attitudes of those who make foreign policy in Washington's closest ally on this side of the Atlantic, Britain.
Britain is supposed to be an island of stability and amity in a European sea of less-firm friends. Prime Minister Margaret Thatcher is called the Iron Lady for her positively Reaganesque opposition to the Kremlin. It is the one European nation contributing to all three legs of the NATO triad - strategic nuclear, tactical nuclear, and conventional defense.
But as the United Nations General Assembly session moves forward in New York, her man there, Foreign Secretary Francis Pym, carries with him a surprisingly different view of Moscow than the one the White House currently holds.
Mr. Reagan, in his activist way, wants to keep on doing things to the Soviets. He wants to hammer away to stop the Soviets from interfering in Poland, to persuade them to get out of Afghanistan, to keep them well out of the Middle East.
His difficulty in Europe is that not even Mr. Pym, one of the heavyweights of the Thatcher Cabinet and a quintessential product of the establishment here (Eton, Magdalen College, Cambridge, 9th Lancers in World War II), sees the Russians as the kind of instant global threat that Mr. Reagan does.
Sitting in his cavernous office off Whitehall the other day, rich with memories of Asia and Empire, Mr. Pym insisted that the US-Europe alliance was still strong and in good health.
He fully shared the US distaste for the Communist Party's dictatorship, and for its expansionism. The Soviets were in the deepest kind of trouble in a number of areas. Internal dissidents and some other Russians had such a lack of confidence in their system that they sought contacts between East and West. Economically and ideologically, the Soviet experiment had failed and it was up to the Kremlin to begin accepting the ''normal rules of international behavior.''
So far, so good, a Reaganite might feel. But generalities are one thing. Specifics are another.
West Europeans believe they need to trade with Russia and the East as a matter of necessity as well as a practical matter of policy toward countries sharing the same continent. Whether Social Democrats or Christian Democrats govern in Bonn, the German policy of wholesale trading with the East is likely to continue. France and Italy look to expanding trade. Huge contracts to supply equipment for the 3,500-mile trans-Siberian natural gas pipeline are too lucrative, and generate too many jobs, to turn down.
Yet Mr. Pym's well-rounded words reflected other aspects of unhappiness. On the Reagan pipeline sanctions, many Europeans believe Mr. Reagan has no right to tell them what to sell to when he is busy supplying the Soviets with American grain. They object to Mr. Reagan asking their companies to break existing contracts.
What bothers Mr. Pym especially are two things.
First, Washington and Europe still have not worked out a common framework, or philosophy, toward using East-West trade as a diplomatic lever against Moscow - and no such approach can emerge if Mr. Reagan keeps on producing sudden policies out of his hat, without consulting anyone first.
Second, the sanctions policy is irrelevant to its stated aim of softening the Soviet hardline in Poland. ''We must be sure,'' Mr. Pym remarked, ''that the disaster in Poland does not become a disaster for the West.'' Unless the issue is solved quickly, in other words, he fears it could divide the alliance even more deeply.
Further than that, Mr. Pym would not go in public. But if you talk to senior British officials familiar with his thinking, the differences with the US are even clearer.
The Soviet Union, these officials believe, is doing so badly in the world that heavy-handed action could push it into even tougher actions that the West would like much less.
The Reagan view is that the cold, calculating, cynical Kremlin is on the march in the propaganda and diplomatic battlefields, ever alert to gain from Western failures.
The British view is that the Kremlin certainly retains awesome physical power , but that its economic and ideological back is very much to the wall.
In conventional arms, Soviet planes and anti-aircraft missiles used by the Syrians in Lebanon have been destroyed by American weapons in the hands of the Israelis. Afghanistan remains a Soviet quagmire.
Push Moscow too far, the view goes, and it is likely to crack down harder in Poland. Paradoxically this approach actually brings British and US officials closer on grain sales: British officials wonder if cutting off sales of food might have a deep psychological effect on the Kremlin and cause a backlash reaction no one wants.
The British see no real way to help Poles resist their military government. The British rule out force, and their leverage with Moscow is limited. But where they differ from Mr. Reagan is in believing that it might well be better to do nothing than to take actions, such as the pipeline embargo, that, in their view, could make the entire Polish situation worse.
What, then, is the British answer to the pipeline impasse, which symbolizes current disagreement?
Briefly, it is that Britain, France, West Germany, and Italy, all with pipeline contracts, must sit down quickly with the Reagan men and develop an overall East-West trade philosophy.
Mr. Pym has wanted to do this since early September, but the French, among others, have blocked him. The French have huge contracts of all kinds with the Soviets, and brook no interference in fulfilling them.
Once the approach has been settled then the British would try to fit into it such specifics as the following:
* Credit. How much to lend to Moscow, and on what terms? How big the down payments? How long the repayment periods, and at what interest?
* Energy. The Reagan team is unhappy at West Germany being dependent for 30 percent of its natural gas on a Soviet pipeline that could be turned off for political reasons. The US would like to sell coal to Europe. Europe replies that it knows what it is doing, that the Soviet gas - and other fuels - will be only five percent of Germany's overall energy mix, and that US coal is too expensive.
* Agricultural exports. Should the US really be selling grain? How to balance domestic political needs? Europe needs jobs and cash from the pipeline. The US has a President with political commitments to his farmers.
* Cocom. Should the list of items considered strategic and banned from export to the Soviet bloc be tightened?Only in dialogue and discussion can such issues be solved, the British believe. But as long as the Reagan administration sees the Kremlin with horns, and Europe sees it as a foundering giant, solutions won't be easy to reach.