With Britain expected to decide next week whether to withdraw from the United Nations Educational, Scientific and Cultural Organization, Prime Minister Margaret Thatcher today said many of the criticisms of that controversial body are "abundantly justified."

Thatcher specifically referred to criticisms about "the direction" of the U.N. agency's expenditures "and to the attempts they make from time to time to prevent the freedom of speech and freedom of the press in parts of the countries of the world."

The prime minister was responding to questions in the House of Commons today from Labor Party member Tom Clarke, who opposes any British pullout. He urged Thatcher "to respond just this once to the Third World and the nonaligned nations instead of hanging on America's coattails."

The United States gave its one-year notice of withdrawal a year ago next month. While the United States contributes about a quarter of UNESCO's annual budget, and the British less than 6 percent, the British carry considerable influence as head of The Commonwealth.

Late last week, 41 commonwealth countries urged London not to follow the Reagan administration in a pullout. Rather, they urged Britain to remain a UNESCO member and lead the battle for reform from within the Paris-based organization, which is widely perceived by both the Reagan and Thatcher governments to be antiwestern.

High-level sources here said the preponderance of opinion within the Foreign Office was for Britain to give the one year's notice of withdrawal and use that decision as leverage for internal reform.

Last April, the Thatcher government warned Amadou Mbow, UNESCO's director-general who is also a major target of western criticism, that unless there were substantial improvements, London would reconsider its membership.

The feeling among officials is that Britain probably will decide next week to give the formal one-year notice of withdrawal. They also said they would not exclude the long-shot possibility of some coordinated move with the United States.

In such a scheme, Washington would delay its actual pullout to bring it in line with Britain's formal notification so the two countries could work together within UNESCO, presumably with even greater leverage as a twosome, to bring about internal change.