Over the long term, the most interesting result of Britain's election may be that the fledgling Alliance of Liberals and Social Democrats did so well in positioning itself to become the country's main alternative to Margaret Thatcher's Conservatives.

The Conservative swing that carried Thatcher back to power will eventually turn in the other direction, as political pendulums invariably do in well-ordered democracies. And when that happens, it now seems a reasonable forecast that the Alliance and not the troubled Labor Party will be the gainer.

Practitioners of psephology, which is what the British call the science of analyzing vote totals, have mostly completed their dissection of the numbers from the balloting. Their findings are notably favorable to Alliance hopes. They show that Liberal- SDP candidates finished second to the Conservatives in 312 seats, whereas Labor came in second in only 120.

Labor did so badly in 119 seats that it lost the small deposit paid for local campaign processing. The Alliance lost only 11 deposits. In the entire southern quarter of the country, Labor did not win a single seat.

In the popular vote, Labor, with 28.2 percent, did only slightly better than the Alliance, which got 25.9 percent. But as Labor votes were concentrated in areas of the north, the party won 209 places in Parliament under Britain's system, in which each seat is a separate race and only winning counts. Because the Alliance votes were spread more evenly and in different sections, it won a mere 23 seats.

Still, according to Robert Waller of Oxford's Magdalen College, the results mean that nearly three-quarters of the conservatives' 397 MPs faced a stiffer challenge from the Alliance candidates than they did from Labor--and, significantly, that this trend appears certain to continue.

Labor's support, Waller says, is increasingly drawn from those sectors of the population whose size is irreversibly dwindling-- workers in the declining manufacturing industries, inner city dwellers and renters. For instance, in 1959 about 75 percent of the voting population was classified as working class. Now it is under 50 percent.

By contrast, the Alliance did well among the expanding middle class, among people who own their own homes (now a majority) and among those whose basic sense of social justice used to make them compatible with Labor but no longer does. Labor's doctrinaire socialism and its unilateralist defense strategy is out of phase with these small "l" British liberals.

No less than a historic realignment in British politics was the dream of SDP founders when they bolted from Labor in 1981. After an initial burst of public enthusiasm, the party faltered for a number of reasons: a fuzziness of policy objectives, limp leadership from Roy Jenkins, squabbles with the perenially third-place Liberals as they forged a centrist compact.

Those were real problems, and many still remain. This election could hardly be classified as an unalloyed triumph. After all, three-quarters of the electorate did not vote for the Alliance, and the SDP lost 23 of its seats in Parliament (mostly the ones held by Labor defectors).

The fact remains, though, that the surprisingly broad reach of Alliance support and its clear demographic advantages over Labor give it a better shot at future growth than looked likely before the election. All the more so as Jenkins has been replaced by the far more dynamic David Owen.

Labor is also due for new, younger leadership. But recreating its image as a party to embrace the full range of center-left sentiment will require an ideological reversal much greater than, say, the Democratic Party went through after the trouncing of George McGovern in 1972. Considering that on the whole, the SDP-Liberal platform in this election was nearer to the Conservatives' than to Labor's, attracting Alliance supporters will not be easy.

For Americans, the interesting point in all this is that it has parallels in our politics beyond the usual Anglo-American comparisons of Thatcher and President Reagan.

In composition and temperament, the Alliance resembles the so-called neo-liberal inheritors of the old Democratic coalition now bidding for dominance in the party. "Instinctively, they measure the well-being of people in terms of state intervention," wrote Ronald Butt in the London Times. They recognize that socialism is not the solution, yet still lack a clearly defined alternative.

On foreign policy, they have mainstream views on the needs for maintaining an adequate nuclear as well as conventional defense. But they are uneasy with the conservative fundamentalism that currently inhabits the White House and 10 Downing Street. Here again, however, they have yet to put forth a fully developed strategy for changes that would clearly define their objectives to confused voters.

The Alliance plainly has a good chance for success in Britain's next national election (or maybe the one after). So do the Democrats in 1984. In both cases, the main challenge is still to figure out the best use of that potential.