When I left London 10 days ago, the wave of riots that swept across the cities of Great Britian was just beginning. But a junior member of Margaret Thatcher's Conservative Party majority in Parliament made the observation, "This is what happens when you separate economic theory from social policy and pursue the one at the expense of the other."

An American journalist, returning to his own country at the beginning of the long, hot summer, cannot help but wonder what warnings there may be for us in the calamities visiting Britian.

The Reagan administration says there are none. Secretary of Treasury Donald T. Regan told questioners on CBS' "Face the Nation" not to "push the parallel," adding that, "You can have civil disturbances in any kind of economy."

He is right. The last round of serious urban riots in the United States occurred just as the Great Society, the last full flowering of welfare-state liberalism, was coming into bloom. Obviously, there is no direct relationship between the growth rate in the federal budget and the tranquillity or hostility of the city streets.

But it would be naive to think that in times of social and economic stress, such as ours, perceptions about the attitudes of the rulers do not influence the behavior of the most miserable of the ruled.

For all their professions of compassion, Thatcher and her associates in the Tory government are seen by many in Britian -- including some not-so-secret critics in their own party -- as economic ideologues who will govern by their monetarist doctrines, no matter what the consequences in Brixton, or Manchester or Liverpool.

Ronald Reagan is a far more affable personality than Thatcher; his economic theories are not as one-dimensional; the American economy is far healthier than Britain's; and our unemployment is one-third lower.

Yet the public opinion polls measure a growing belief that Reagan's economic policies are harmful to the elderly, harmful to the poor, harmful to minorities. And that suspicion is sharpest among those who are living on the margins of the economy, out of work or working for subsistence wages.

Look back at the report of the Kerner Commission, which conducted a massive study of the causes of our 1963-67 urban unrest. In the chapter on "the basic causes," the commission described the factors of migration, discrimination and segregation that led to the existence of the black ghettos -- conditions that have not changed that markedly in the 13 years since the report appeared.

And then it said that "recently, three powerful ingredients have begun to catalyze the mixture." Those ingredients were "frustrated hopes, the legitimation of violence and a sense of political powerlessness."

Can we honestly say that those factors have diminished today?

In the intervening years, blacks have come to power in many major cities, from Atlanta to Los Angeles. Millions of black youths have achieved the dream of higher education and are making their way into the middle class. The "violence" of which the Kerner Commission spoke -- white police attacks on black civil rights demonstrators -- has been ended.

But the violence of crime is an ever-present factor in the ghetto. Hopelessness still dogs the 20 to 30 percent of big-city minority youths who cannot find their first jobs. And throughout the black community, at all levels, there is a sense of exclusion from the decision-making of this government greater than I have known in 20 years -- a real sense of being the impotent outsiders.

A reader in Minnesota recently sent me an editorial from the Princeton, Minn., Union-Eagle, a weekly newspaper published by Elmer L. Andersen, the former Republican governor of that state.

"Can we say there is an even-handed application of a new fiscal policy to reduce government spending." it asked, in appraising the Reagan record so far. "No, there is not. What is clear is an enormous shift in govenment spending from social services to military spending, and no indication that the result will be balanced budget for many years to come. . . . Furthermore, there is a harshness about action and attitudes in the social-services area that is not evident toward excesses in military spending. We are going after school-lunch programs, food-stamp distribution, aid to families with dependent children, grants-in-aid for the arts and humanities, with crusading vigor."

If that is the way it looks to Elmer Andersen, a good Republican, in Princeton, Minn., how can it possibly look to Joe Jones on the South Side of Chicago?

Britain is offering us a grim reminder that people are not laboratory animals, available for economic experimentation. When they reach the breaking point of frustration, they rebel. I pray we do not have to relearn the lesson here.