BRITAINS'S great economic experiment appears to be moving toward a critical turn. If it does not begin to pay off visibly and handsomely within the next few months, political reality is very likely to force its abandonment. Business people, crucial to Margaret Thatcher's Conservative government, are beginning to edge toward the fire escape. Unemployment is now the highest since the Depression. Inflation is down a little, but still the highest in any major industrial nation. The rercession in Britain is going to be deeper and much longer than here.

The British are engaged in an interesting test of a monetary theory holding that the inflation rate must fall when the government restricts the money supply. Labor is forced to accept smaller wage increases, according to the theory, and the resulting price stability will send both production and employment strongly upward. The experiment has now been running 15 months. It's already clear that peculiarly British conditions -- above all, notoriously bad labor relations -- mean that the outcome may not be entirely applicable to other countries. But there are a couple of points that deserve careful consideration by those Americans who might be inclined to try similar monetary policies.

In a time of floating exchange rates, domestic economic policies sometimes work quite differently from the ways in which economists expect them to. To restrict the money supply, the Thatcher government has resorted to very high interest rates. That raises the exchange rate of the pound. The North Sea oil also tends to raise the pound. That makes British exports expensive and hard to sell abroad. Normally, a country's export industries are its most innovative and productive. The extraordinarily high exchange rates are now strangling precisely those kinds of industries in Britain.

Monetarist theory has also run into trouble in the labor markets. In Britain's nationalized industries -- like steel, coal and electric power -- the government itself is the employer. Wages there continue to be set through an exercise of the government's political judgment, and settlements tend to be significantly higher than in a competitive market. There's a strong American parallel, incidentally, in the process responsible over the past decade for the very fast runup of steel industry wages here.

If the monetary experiment fails this winter, what comes next? The attempts to control inflation keep coming back to the question of wages in those economies and those industries -- not only the British, and not only steel -- in which wages are now regarded as a political issue, to be settled by politicians.