British voters decide Thursday whether they want to continue the Labor Party's steady expansion of the welfare state or turn to the right with Margaret Thatcher's Conservative Party.

Three new public opinion polls to be published in Thursday morning newspapers here show the Tories' reduced lead over Labor levelling out at about 6 percent, which lends support to Thatcher's "cautious optimism" that the Conservatives will win a working majority in the 635-seat House of Commons.

During the campaign's closing days, Thatcher has been appealing to undecided voters to help give her a large enough parliamentary majority to implement reductions in income taxes, government spending, labor union power and government intervention in the economy. She said this will "restore freedom" and "make Britain great again."

Prime Minister James Callaghan, meanwhile, seemed confident that his greater personal popularity and stately, low-key campaign-defending the accompliishments of democratic socialism and warning of "change with chaos" from the Conservatives-would yet produce an upset victory for the Labor Party.

The latest polls, completed yesterday by the Marplan and MORI polling firms for the Sun and Daily Express newspapers, both show the Conservatives holding a 6 percent lead over Labor. A Gallup poll completed yesterday for Thursday Daily Telegraph shows the Conservative lead down further to just 2 percent among prospective voters and about 5 percent among those who say they "definitely" will vote.

The polls show that Labor's greatest strength is in Scotland, northern England and Wales, while the Conservatives are strongest in populous southern England and the industrial Midlands, which could win them more seats in Parliament than their slim overall lead in the opinion polls suggest.

The Conservatives' strength in the polls has been declining steadily, while Callaghan's lead over Thatcher in personal popularity showed a rapid increase to a nearly 2-to-1 margin in most of the polls.

Most of the erosion in Conservative support has not gone to Labor-which has has not measurably increased its strength during the month-long campaign-but do the Liberal Party, which has roughly doubled its support in the opinion polls during the campaign to 13 percent in all three of the new polls.

Although a vote that size on Thursday would be substantially less than the 18 percent the Liberals won in the last national election five years ago, Liberal leader David Steel said his party's strategy of targeting key constituencies should increase its seats in Parliament from 14 to more than 20 and hold the balance of power if neither major party wins an overall parliamentary majority.

"If no party gets over 318 seats, no party has won," Steel told reporters yesterday. "The people has won."

For a stable government to be formed, Steel said, either Labor or the Conservatives would have to form a coalition with his party, agreeing to some of its demands for change. These include proportional representation (which would greatly increase the Liberal's strength in Parliament), reform of the now unelected House of Lords, extensive tax reform, and limited home rule for Scotland (Steel's home), Wales and the various regions of England in federal system.

The Liberals, under Steel's leadership, had supported the minority Labor government in the informal "Lib-Lab pact" during two years of Callaghan's administration, but Steel said he would now be seeking something more formal, lasting and beneficial to his party.

Steel has been offering undecided voters what he calls "insurance" against the "extremist" policies of Labor on the left and the Conservatives on the right. The Liberals, he said, could stop Labor from nationalizing more industries and "kow-towing" to labor unions, and could prevent the Conservatives from "cutting into the muscle of the welfare state."

The Liberals may have trouble holding onto all of their present seats, however, as might two other minor parties. The Scottish and Welsh nationalists. Throughout mainland Britain, the strength of the minor parties has fallen off noticeably in the polls from the last national election in 1974.

Across the Irish Sea in Northern Ireland, all 12 Ulster seats in Parliament will remain in the hands of local sectarian parties, most of them in one faction or another of the predominantly Protestant Ulster Unionists. They want Ulster to remain part of Britain. But they also want a return of home rule without the Protestant majority sharing power with the Catholic minority.

A "hung Parliament," in which neither the Conservatives nor Labor had an overall majority and the rest of the minor parties held fewer seats than they do now, would give the Ulster Unionists more leverage.

Irish-American politicians led by House Speaker Thomas P. (Tip) O'neil Jr., as well as "Troops out of Ireland" demonstrators-apparently including Labor Party members-have heckled Callaghan around the country. They argue that Northern Ireland should have been an issue in the campaign. They say both major parties have favored the Protestant majority, while maintaining the status quo with British toops in Northern Ireland to curry favor with the Ulster Unioinists.

After O'neil charged during his recent visit to Ireland and Ulster that the British had made Northern Ireland's fate a "political football," both Callaghan and Thatcher answered that it was a bipartisan matter that should not become a subject of campaign rhetoric. Both said they would seek a political solution acceptable to both Protestants and Catholics there.

Militant factions of the Irish Republican Army, who had vowed to disrupt the British electionwith violence, assassinated the Conservatives' Northern Ireland spokesman, Airey Neave, at the start of the campaign and have since shot or murdered with bombs 15 people in Northern Ireland, most of them Ulster security personnel. More than 50 other people have beeninjured and buildings have been damaged by bombs and fires during the IRA campaign of violence.

As of tonight, the IRA had not struck again insider mainland Britain since Neave's assassination. But his death and the tight security that was quickly thrown up around leading candidates-similar to presidential security in the United States-limited the access to Callaghan and Thatcher in particular and seemed to dampen the spirit of the campaign.

Most observers here found it an unusually quiet if not dull campaign. Thatcher observed this week, however, that the voters seemed to focus more seriously than usual on the issues because the two major parties were offering such a distinct choice in what Callaghan has repeatedly called "an important election that will decide which way we will go in the 1980s." CAPTION: Picture; MARGARET THATCHER. . . expresses "cautious optimism"; Picture 2, JAMES CALLAGHAN...looking for an upset