In presenting the new Conservative government's plans for sweeping economic change in Britain Tuesday, Prime Minister Margaret Thatcher made clear that she is not about to compromise her electon campaign crusade against socialism now that she is comfortably in office.

Opposition Labor Party politicians and even prominent pro-Tory journalists here had been predicting a more timid launching of the Conservative ship of state once Thatcher came face-to-face with what they call "the realities of government."

But in her vigorously delivered speech to the House of Commons-punctuated by the familiar, extemporaneous, anti-socialist rhetoric of the campaign-Thatcher demonstrated her determination to move immediately on everything she had promised the voters.

Taxes and public spending would be cut. Government intrusion in the economy would be reduced by selling nationalized businesses back to private investors, abolishing the price commission and repealing other laws that businessmen had found to be a nuisance. A fourth television network would be used for commercial broadcasting rather than more public television. Private schools, private health care and private home ownership would be encouraged as alternatives to comprehensive public schools, the national health system and public housing.

Thatcher did not back down from any potentially nasty fights. Legislation to curb union power would be introduced, even if prior consulation with the unions failed to win approval for it. Black and Asian immigration would be further restricted, even if that meant keeping out parents, husbands or adult children of immigrants already settled here.

Thatcher briskly brushed aside taunts and criticisms of her proposals from the opposition Labor benches by declaring that "the path we now follow is the path the people have chosen. We offer the people of this country the policies they instinctively favor and desire."

This same confidence and certainty has been characteristic of Thatcher's approach to her new job. She has dominated her unusually brief Cabinet meetings as recent prime ministers seldom have, replacing the traditionally collegial discusions of policy with rapid, strongly led decision-making. She is personally overseeing the preparation of her government's first budget, which will be presented to Parliament June 12.

Although legislation she proposes is virtually certain of passage by the Conservative majority in the House of Commons, Thatcher will still need all the determination she can muster to make changes that cannot be accomplished by legislation.

One such problem is controlling inflation, which she set as a top priority. Rising food prices plus high wage settlements during the last month of Labor government have pushed inflation back into double digits, to 10.1 percent this month. It is expected to rise to 12 or 13 percent this year before leveling off.

Teachers, power workers, civil servants and other unionized workers are still seeking pay raises of up to 35 percent in smoldering labor disputes that could become crippling strikes. Thatcher did not endear herself to the teachers, who are refusing to do nonclassroom work until they get more money. When she said in her first public speech as prime minister last week that "it cannot possibly help the honorable profession for teachers to damage the prospects of children they teach."

It will take time to reduce the government payroll and decide where cuts can be made in the government subsidies for business, housing, transportation and regional development without slowing the economy Thatcher wants to accelerate.

Despite its comfortable control of Parliament, Thatcher's Conservative government also finds itself in confrontation with Labor-controlled local governments in many big cities. They will fight Thatcher's plan to sell hundreds of thousands of public housing units to their tenants.

Thatcher's team at the treasury is also finding it harder to cut government spending than first expected. Large increases in spending and public borrowing were left behind by the outgoing Labor government. And Thatcher has quickly given big raises to police and soldiers on top of that.

Another example is Thatcher's desire to stop the merging of elite public schools for bright students-like the one that prepared her for Oxford-into comprehensive schools for students of all abilities.

A bill to repeal the Labor government legislation that required the mergers will be introduced in Parliament Thursday, and Thatcher's education secretary has already stopped the process administratively. But Labor-controlled local education authorities cannot be compelled to bring back elite public schools.

While the election clearly demonstrated the popularity of Thatcher's policies, as she reminded the House of Commons Tuesday, opinion polls showed there was still considerable uncertainty about her ability to lead the entire nation rather than just her party. It is this challenge that she faces in tackling problems and making changes outside the now friendly confines of the Commons chamber. CAPTION: Picture, MARGARET THATCHER . . . socialism her target