She did it again!

AS expected, British Prime Minister Margaret Thatcher has been re-elected for a third term - a feat not achieved by any of her predecessors for a century and a half. What was not expected was that she would do as well as she did. She went into the campaign last month with a majority of 137 seats; the consensus was that the elections would leave her majority cut to some 40 or 50. In fact, her majority looks to be better than twice that.

Such figures, however, can misstate her popular support as badly as the Electoral College tally misstates the mandate of an American president. Mrs. Thatcher's Conservative Party won only about 42 percent of the vote.

And it was not only the votes the Tories won but those that Labour lost that mattered. Neil Kinnock has been an engaging and forceful leader, who deserves credit for revitalizing the Labour Party. But Labour's defense policy - calling for unilateral nuclear disarmament of Britain - put voters off. The considerable efforts of the party's campaign - including a 10-minute video on Mr. Kinnock that was proclaimed an instant advertising classic - weren't enough.

It was also a dark election day for those who had hoped the Alliance, a coalition of the Liberal and Social Democratic Parties, would overtake Labour as the primary opposition in Westminster and provide a more centrist approach to government. Three of the Social Democrats' founding ``Gang of Four'' failed to win reelection, and David Owen, their highly regarded leader, is left looking like a man without much of a party.

And so the Thatcher victory reinforces Britain's tendency at this time toward polarization, rather than consensus building at the center, and to split into two nations: the haves and the have-nots. The split is geographic as well as economic: the Tory South and the Labourite North.

Some of the economic achievements of the Thatcher years have been more apparent than real. A case can be made that Britain today is a land of falling inflation and joblessness, and rising productivity and economic growth - but only by careful selection of statistics. Thatcher's main achievement may prove to be her cheerleading for the free-enterprise system, her talking up the value of hard work and thrift and all the other virtues she learned growing up as a grocer's daughter in Lincolnshire.

Her foreign-policy achievements have been considerable. Most notable recently have been the Anglo-Irish accord, which has strengthened the moderates in Northern Ireland, and her successful visits with Soviet leader Mikhail Gorbachev.

With most of the other major Western leaders either lame ducks or in electoral trouble, Thatcher becomes the senior leader of the West. The continuity she will be able to provide among the economic ``Big Seven'' as well as within NATO will be valuable.