ONE SPECIAL election hardly constitutes at trend. But the election of Shirley Williams to Parliament, from a suburb of London, offers a highly interesting signal. It strengthens the impression that some of Britain's voters are getting deeply uneasy about the unaccustomed movement of their traditional political parties toward harshly doctrinaire positions on the right and left. In at least this one constituency, people have begun to look for the center. It's the center, or perhaps center-left, that Mrs. Williams and the new Social Democratic Party, in alliance with the old Liberals, now represent.

The politics of frustration has trapped Britain for a decade--frustration mainly over poor economic performance and the inability of a succession of governments to do anything about it. As Britain slid farther behind the stronger economies of France and Germany, and as all the usual economic prescriptions proved futile, strange things began to happen in both of the established parties. The past several years have produced both Margaret Thatcher's rigid monetarism and an opposition that is increasingly explicitly socialist, isolationist and pacifist.

The same frustration, the product of inflation and low economic growth, has spread widely throughout Western Europe. Perhaps, in view of the Polish turbulence, you have to add that it's not limited to the West. France has turned in exasperation from a succession of conservative governments to one that is, at least in domestic affairs, very much on the left. In equal exasperation, Sweden has done just the reverse. In Germany, the strains are visible, not among the parties, but within them. But it's only in Britain that there is now an explicit attack on the party structure.

The people who voted for Mrs. Williams evidently felt that the choice offered by the two-party system was growing dangerously wide. At least in that consitutency, Mrs. Williams and the new party clearly represented a more tolerable compromise between the benefits of economic growth and the benefits of government-guaranteed security. Not only in Britain but throughout the industrial world, the rapid increases in prosperity in the 1960s and early 1970s generated public standards and expectations that succeeding years have been unable to sustain. Readjustment--not to lower standards of living, but to a less dramatic rise--is a process that has already devoured dozens of eminent political careers, by no means all of them British. Mrs. Williams' victory hints at the formation of a constituency that accepts what seem to be the economic realities of the 1980s--a decade of growth, but growth that is neither so steady nor so easy as the past two decades' rhetoric had led voters to believe.