Eighteen months before Ronald Reagan's landslide victory, Margaret Thatcher's Conservatives won a strong mandate here to make drastic changes in economic policy strikingly similar to those since promised to Americans by Reagan and his advisers.

But the Thatcher government has conspicuously failed so far to make those policies work, while the ailing British economy has gone from bad to worse. Although Thatcher publicly insists her strategy is Britian's only hope for eventual improvement and recovery, her government is being forced by gloomy realities to make significant changes in course and abandon some of its monetarist theory.

Like Reagan, Thatcher planned to make a series of dramatic tax cuts to get the British economy moving again.

But the first income tax cut just after Thatcher took office was also the last for the foreseeable future, and it has been wiped out for all but the highest income tax payers by steep increases in national sales and excise taxes. Even worse, it is now expected that more tax increases will be necessary next year to reduce a growing government budget deficit.

Analysts are quick to point out significant differences between the American and British economic and political systems. Britian's economy is much smaller, less varied, more dependent on manufacturing and export sales. It is also more dominated by government, from the public housing provided for one British family in three to the government-owned auto, aerospace, airline, railroad, coal and steel industries. g

An additional important difference is the semi-independent status of the U.S. Federal Reserve System, which over the past two years has restricted the money supply in an effort to curb inflation. The Fed did so with implicit support of the Democrats controlling the White House and Congress, presaging policies advocated by both Thatcher and Reagan.

Nevertheless, although nearly all of Britian's problems are more severe (except for its plentiful supply of oil from the North Sea), both countries are suffering in similar ways from economic stagnation, low productivity, and high unemployment, inflation and interest rates.

Like Reagan, Thatcher had originally intended to make big tax reductions possible by slashing government spending, except for a sizable increase in spending for defense.

But even after deep and politically unpopular cuts in housing, education and social welfare programs, her government has found it impossible to reduce overall spending by even half as much as planned so far. And to cut even that much, it is being forced to bring defense spending down below its planned increase of 3 percent above inflation, despite previous Thatcher commitments and the strong objections of Britian's military chiefs.

Like Reagan's monetariest economic advisers, Thatcher's doctrinaire Treasury team had expected to squeeze down the inflation rate by reducing government budget dificits and tightly controlling the growth of the country's money supply.

But the money supply has ballooned out of control, because the Treasury and Bank of England apparently used the wrong techniques and both the government and desperate private businesses have been borrowing more rather than less money. Near-record lending rates appear to be only compounding the problem by forcing cash-starved businesses to continue borrowing still more in rapidly turned-over loans with high interest charges.

The inflation rate, meanwhile, more than doubled under Thatcher to a peak of 22 percent last summer before falling steadily since to just over 15 percent today. Most analysts believe it has been Britian's severe recession, rather than the government's monetarist policies, that moderated price and wage increases by battered private industry.

Perhaps most important, like Reagan and his advisers, Thatcher and her economic lieutenants expected their policies to create an invigorating new climate here in which the government's role in the economy would shrink and private enterprise would be freed to grow and become more productive.

But private industry has so far suffered disproportionately instead. Its sales, profits, wages and unemployment all have plummeted alarmingly despite Thatcher's abolition of restrictive banking, investment, dividend and price controls and reduction of government-required paperwork.

Some businessmen acknowledge their firms belatedly are being forced to become leaner and fitter. But many of Britian's largest corporations have suffered unprecedented losses and are shutting factories, while countless smaller firms, efficient and inefficient alike, face complete collapse or have already closed their doors.

Unemployment has continued rising to near-depression levels never expected in Britian's postwar welfare state, which could pose dangerous political and social problems for Thatcher. The new opposition Labor Party leader, Michael Foot, a veteran protest march participant and polemicist, has vowed to mobilize nationwide pressure against the government, beginning with a demonstration against unemployment in hardhit Liverpool later this month.

In the face of all this, Thatcher's government has been forced to defy its own rhetoric by increasing subsidies to nearly bankrupt nationalized industries and planning an expensive package of emergency government aid for the unemployed. It also has abandoned its monetarist hands-off policy on wage negotiations to try to restrict pay raises for government workers this winter. Her drive to eliminate unneeded government employes has so far been modified by a dug-in civil service to not filling vacancies when they occur in some departments.

"Government policy has not worked out as it intended or as industry expected," complained British Aluminum chairman and Confederation of British Industries official Ronald Utiger, echoing the criticism of many British industralists. "A government which set out to reduce the public sector is in fact reducing the private sector."

Thatcher aides admit that the government's failure to do better on its key pledges -- to control the money supply, cut government spending and reduce taxes -- has undermined its credibility and given its monetarist theories a bad name, particularly among its business and Conservative Party supporters.

"I cannot recall any time in our history when so much damage has been done in the name of an unproven nostrum," declared Sir Emmanuel Kaye, chairman of Lansing, Bagnall Ltd., at a recent meeting of business leaders.

Milton Friedman, the American Noble Prize economist, along with other academic monetarist in Reagan's entourage, has recently been trying to dissociate himself and his theories from the "Thatcher experiment" that they had been watching closely as a laboratory test of monetarist theory.

Monetariests who had thought Thatcher would show the United States how their theories could rescue an economy from these problems are now accusing her government of failing to give monetarism a fair test even though her Conservatives have complete control of the British Parliament by a 43-seat majority.

Friedman, who has not been in Britian for nearly nine months, blames the British Civil Service and the Bank of England for either thwarting Thatcher's monetary policies through bureaucratic inertia or executing them ineptly by trying the wrong techinques to control the money supply.

"There has been significant movement in British policy," Friedman said in an interview broadcast here by the BBC, "but it has not been as extensive as would have been desirable even in the area of monetary policy or equally in government spending.

"If [Thatcher's] announced policy could be made effective and really carried out, the transition period would be at most a year. Then you would see inflation rates coming down and real growth going up simultaneously."

The influence of Thatcher's monetarist Treasury ministers also is ebbing inside her Cabinet. They are now being forced to modify proposed severe cuts in defense and social welfare spending, after a number of other Cabinet members objected strongly.

A proposal by the Treasury minister to prevent old age pensions (equivalent to Social Security benefits in the United States) from rising as much as the inflation rate was reportedly ruled out by Thatcher and after her most senior and trusted ministers, Deputy Prime Minister William Whitelaw and Lords Soames and Hailsham, pointed out that she had publicly promised that the pensions would be completely protected from inflation throughout her government.

It now appears likely that Thatcher will have to settle in next year's budget for much smaller government spending cuts than she sought, significant increases in taxes, and a larger budget deficit. Her government also is expected to allow the minimum lending rate to fall after more of this winter's wage settlements have been reached.

"We finally have real Cabinet government now," one well-informed Conservative member of Parliament said of these collegial decisions. In his view, theoretical monetarism is being replaced by more flexible, traditional fiscal conservatism in the Thatcher goverment.

Before Thatcher's Conservatives must next face the voters in 1984, most economic analysts expect the country's long, deep recession to have finally ended. They believe fast-rising government revenues from North Sea oil should also make possible further income tax cuts and investment in new high-technology industry.

Businesses and decaying inner city industrial areas may also benefit by then from the government's establishment of special enterprise zones where red tape and possibly taxes will be reduced for new business ventures.

But, besides weathering heavy storms of economic difficulty and political dissent between now and then, Thatcher aides say she faces the problem of how to justify changes in policy she is making without losing the "conviction politician" and "radical" image she has enthusiastically cultivated.

When Thatcher spoke recently on her policies at the annual banquet of the lord mayor of London in the historic Guildhall, she insisted she was not deviating from her charted sources. But individual policies were nevertheless beginning to sound different. And not once during this major speech to British bankers did she mention controlling the money supply, which had been a constantly recurring theme just a few months ago.