PRIME MINISTER James Callaghan has lost his gamble. Last summer he decided no to hold autumn elections, in the hope that his Labor Party's fortunes might improve over the winter. But it has been a wretched winter for the British, endless months of strikes and rising dissension. Then Mr. Callaghan decided against a spring election, in the hope that something might turn up over the summer. But he presided over a minority government, sustained in power only through a succession of fragile deals with the political fragments that have begun to be an important part of British parliamentary life. Now the last of the deals has collapsed. The Callaghan government has been brought down-by a margin of one vote-and the country goes into an election that the opposition Conservative Prty will very probably win.

In that case, Margaret Thatcher will become prime minister. The prospect for any great movement of national reconciliation does not seem, on the whole, very bright. Mrs. Thatcher's supporters defend her by saying that in office she would not prove so rigid and doctrinaire as she might appear at present. That's a thin sort of reassurance.

The crucial test of Mrs. Thatcher's skill will be her relations with the labor unions. The large achievement of Mr. Callaghan's tenure was the successful series of voluntary wage agreements into which he drew the unions. From an inflation rate of nearly 30 percent four years ago, those agreements had brought the country down to a rate of 7 percent last autumn by the time they began to come unraveled.

Mrs. Thatcher's policy toward the unions has been sharply adversary. She promises severe legislation to restrain wildcat strikes. She is counting on a surge of public exasperation with the unions to support her in imposing a new regime on them.

Over the past couple of years the Callaghan government's preoccupation with inflation, and its eroding strength, have led it to shunt aside other pressing issues. The unhappiest example is Northern Ireland, where the terrorist campaigns continue. No one has yet found common ground between the Catholic and Protestant communities there; all the political initiatives of the past decade have collapsed. Losing faith in constitutional reform, the Callagham government relied increasingly, by default, on the police and the troops to deal with the terrorists. But the terrorism continues and , inevitably, the evidence of police brutality is increasing. It would be pleasant to think that Mrs. Thatcher might bring another approach to the ancient divisions of Northern Ireland. But if she has anything in mind, she's keeping it to herself.

Britain, divided and perplexed by its own accumulated troubles, seems to have turned inward. The art of government there has come upon a slack period, as it does occasionally in all countries. As in other arts when the rewards decline, the breadth of inspiration and the quality of performace can be expected to suffer. This kind of decline is not likely to ve permanent. But neither is it likely to be reversed in the months immediately ahead.