MARGARET THATCHER set out in 1979 to change the course of British politics. To a remarkable degree she has succeeded, and now, in another great triumph, she has won five more years for her Conservative party to push the country toward, as she might put it, the virtues of self-reliance and individual initiative.

For the United States, Mrs. Thatcher's victory also means a stable and reliable alliance. In this country it wasn't only the Reagan right wing that found the Labor party's view of the world alarming.

Mrs. Thatcher's style of leadership is, by any standard, pretty aggressive. Her manner is deliberately divisive, and British politics has become increasingly polarized. The Alliance of the Liberal and Social Democratic parties, which stands for the old style of consensus politics, was the biggest of the losers in Thursday's voting. The split in her opposition between Labor and the Alliance, incidentally, has been great good luck for Mrs. Thatcher, enabling her to win large parliamentary majorities with less than half of the popular vote. On Thursday the Conservatives got just over 42 percent of it, very similar to their shares in 1983 and 1979.

The past eight years have been good ones for many of Britain's people, particularly those who work for private companies. But the disparities have been widening. The south prospers while the north is deep in depression. Most of heavy manufacturing keeps sinking while other industries, particularly those involving finance and electronics, do conspicuously well. Unemployment is still around 11 percent; it has been in that range for five years, and there's now a sub-class of the more or less permanently unemployed.

If the British economy is being strengthened, this is being achieved at a substantial cost. But Mrs. Thatcher has unquestionably given much of her country a sense of pride and possibility that was not there when she first came to power. This extraordinary third consecutive victory, a feat unparalleled in modern British history, now enables her to continue her campaign for national renewal.