BRITAIN'S PRIME MINISTER, James Callaghan, only barely survived the latest parliamentary challenge to him. But he survived, and that's the important thing. Whatever the faults of Mr. Callaghan's Labor government, there is little reason to suppose that the Conservative opposition could do as well. Led by Margaret Thatcher, a Conservative government would shortly find itself locked in another wasting confrontation with the unions - just like the last time. As long as the main business of a British government is to preside over a reduction in real incomes, the best leadership is going to be the one that can work most effectively with the unions.

By striking a deal with the minuscule Liberal party, Mr. Callaghan squeaked through Wednesday night's vote of confidence and bought himself he believes, another six months' tenure.If that's true, it's very heartening. Six months would take him through the renegotiation next summer of his compact with the unions to hold wage increases significantly lower than the inflation rate. The unions publicly doubted last year that they would ever renew the compact again. But just as there is no very good alternative to Mr. Callaghan at the moment, so there is no very good alternative to the wagage compact.

The character of British politics is changing, with the emergence of a variety of regional movements - Irish, Scottish and Welsh - that try to use their votes as leverage between the two major parties. Instead of dealing with these regional splinters, Callaghan turned to the Liberals, who agreed to support him on economic issues in return for consultation. But that bargain will now raise the tension between Mr. Callaghan and the left wing of his own Labor party. While Mr. Callaghan's sudden contract with the Liberals has averted a premature election this spring, it does not look like anybody's idea of an enduring alliance.

Parliamentary instability seems to be increasingly the pattern in Western Europe, from London and Paris down to the Mediterranean. The reasons are doubtless related to the end of the long boom - the remarkable surge of economic growth through the 1960s and early 1970s. Europeans have lately been considering the unwelcome possibility that the 1975 recession was not merely a temporary blip in the upward curve, but the end of an era.The oil crisis of 1973-74 played a great part in this sudden collapse of the psychology of plenty. Beyond its direct financial impact, it was a demonstration of a vulnerability that shook Europe badly. If it is true that the good old days of high growth and very low unemployment are not going to return, a long series of social policies and expectations will have to be severely revised. We are now seeing the early stages of precisely that painful transition.