Prime Minister Margaret Thatcher should not be meeting President Ronald Reagan in Washington. Reagan should be meeting Thatcher in London.

There, the president would see the future and could judge whether it indeed works. "Thatcherism" has been in place in Great Britain for two years now, and its similarities to the Reagan proposals are striking. If the results prove similar, the British experience assumes profound importance for the American people.

Thatcher began her term with two great weapons. Her people agreed that the status quo had failed, that change was essential.

Her people also desperately wanted her program to succeed. She had promised tax cuts and spending cuts. She had promised to end inflation, and she believed that the money supply was the root cause of that inflation. President Reagan shares these precepts and has similar support from the American people.

Now, in the language of our new secretary of state, let me "caveat" a bit. Britain is quite different from the United States. It is 10 years farther down that slippery incline that all Western democracies are traveling. It is a society in which class lines are sharper, where people are less willing to move to new jobs, where foreign trade is more important and where disputes between labor and management are more bitter.

Government is bigger, public housing bigger, national ownership and intervention in industry bigger, social programs bigger, jobless benefits bigger.

In short, in almost every way but one, the problems are more difficult. That one area is energy, for with the North Sea fields, Great Britain is self-sufficient in oil and has a resource of such magnitude that it could enable that lovely island to solve its many problems and prosper in the years ahead.

One more caveat. The fate of Thatcherism -- whether success or failure -- will be decided by the British people, not by any visiting American. The House Budget Committee did not go to London recently to judge the British but to learn how their experience might apply to our future.

What did we learn?

Thatcher's policies have provided a rigorous and necessary jolt to her people. Britain must change many of its practices and attitudes, and she has furthered that cause.

Her government also has made progress in cutting, spending, a step that must be taken by all of the Western democracies. Inflation is down to about 8 percent.

President Reagan also can succeed on these points. The U.S. government is taking up too big a portion of GNP. Americans have become too dependent on government. The relationship among lobby. Congress and federal agency had left us apparently incapable of cutting spending.

Ronald Reagan may well have ensured his place in history by destroying that debilitating barricade.

But now for the bad news.

Unemployment in Great Britain is about 15 percent in U.S. terms. A grim adjective is applied to those out of work -- they are termed "redundant." Production fell 14 percent last year, the biggest drop since the Great Depression. Interest rates are very high (though several points below our dreadful rates). Excellent companies as well as inefficient ones are on the verge of bankruptcy.

What has gone wrong?

There was an obsessive attention to monetary targets and the apparent belief that if only the money supply were held to the right target, all else would be well. Monetarists will argue that their theory is sound and that Thatcher has not applied it purely enough. A more realistic view would be that money supply is important, but no more so than many other factors. You do not just take a patient's pulse before operating. You use every diagnostic tool available.

The Reagan economic team seems to have a tendency toward the same overdependence on monetary targets.

The Thatcher government spending cuts were incomplete. She did not deal with indexing, one of the driving forces in inflation in all of the advanced nations.

President Reagan -- who now has nearly overwhelming support for spending cuts -- has also ducked this toughest issue of them all. The failure to address indexing is likely to haunt us all in years to come.

The Thatcher government cut taxes across the board, then raised the value-added tax, which is hardest on the poor and middle-income families. The result was no stimulation for the economy, no growth, no job creation.

The Reagan administration's Kemp-Roth plan also will be plesant for the rich. But the middle class will find its cuts taken up by higher Social Security taxes, higher fuel costs and inflation. There is no indication, let alone assurance, that Kemp-Roth tax savings will go to investment in productive assets.

The extraordinary growth rates upon which the Reagan budgets are based are unlikely to occur, just as they have not occurred in Britain.

And without such growth, the combination of personal tax cuts, major increases in defense spending (Britain also raised defense spending sharply) and the rising cost of unemployment stemming from the impact of high interest rates will result in very high deficits, on the order of $100 billion. Such deficits will drive interest rates up, and government borrowing to cover the debt will absorb money that otherwise would have been available for private investment. The housing and auto industries will suffer badly, and that will ripple through the entire economy.

Last and most dangerous, the Thatcher government is perceived to have overpromised.

President Reagan, as did the prime minister, runs the risk of overpromising.

We are told that Kemp-Roth, monetarism, regulatory reduction and reduced federal government will bring us almost overnight to the promised land of high growth and strong defense, low taxes and low inflation. And it won't even hurt.

When his advisers are asked to explain how that will come to pass, the answer is: because they say so. They have asked the American people for an act of faith, and the American people are granting them that faith.

The administration is offering a miracle cure. But what will happen if there is no miracle cure?

Britain has seen no miracle. Will similar policies produce one in the United States?

To President Reagan's everlasting credit, he has created a climate in which the American people are demanding that the problems be attacked. But what a sad day it will be if in a few years we look back and say, "Lord, what an opportunity we had in 1981. If only we hadn't fallen for theories that made it all sound so easy. If only we had faced up to the problems then and had said there is no magic solution."

Yes, we must cut spending and act to get productivity rising again. Yes, we must strengthen our defenses. No, we can't afford a massive untargeted personal tax cut. We first must get our interest rates and deficits down and our employment and production up. Yes, it will be hard, and it will hurt, and it will take time.