Britain sought to reassure black African Commonwealth states today that the Reagan administration has modified its policy on southern Africa and given key assurances that it will support international efforts to secure independence for the territory of Namibia.

Officials participating in sessions of the 41-member Commonwealth, including representatives of African states, said the apparent shift in U.S. policy on the issue has significantly improved prospects for implementation of U.N. peace proposals and an end to a protracted guerrilla war for independence of the territory, which is currently ruled by neighboring South Africa.

The U.S. assurances reportedly emerged in a meeting in New York late last month of a five-nation "contact group" formed as a go-between in negotiations to solve the issue of Namibia, formerly called South-West Africa. Britain and Canada, both members of the Commonwealth, are part of the contact group, along with the United States, France and West Germany.

Specifically, the Reagan administration is said to have dispelled doubts about its commitment to a U.N. Security Council resolution on Namibian independence by pulling back on its implied linkage between a solution in Namibia and withdrawal of Cuban troops from neighboring Angola. The United States also agreed that South African troops in the territory must be removed during implementation of a peace plan, according to delegates of the Commonwealth of Nations conference here.

In a briefing on the Namibian issue today, British Foreign Secretary Lord Carrington said repeated statements made by group members after the Sept. 24 meeting that efforts to achieve independence were "back on the rails" and recent talks between U.S. and South African officials on the subject "give grounds for qualified optimism."

African delegations, some of whom had harshly criticized the administration for its increasingly "friendly" attitude toward South Africa, expressed interest in Carrington's remarks, and said they would wait for the results of the new Reagan policy emphasis.

"I'm pleasantly surprised to note the public expression of policy from Washington according to Lord Carrington," said Abdul Minty, an anti-apartheid lobbyist and adviser to the SWAPO (South-West Africa People's Organization) representative attending the conference as an observer.

In his briefing, Carrington said, "there is no question of the withdrawal of Cuban forces from Angola being a precondition of independence. But -- there is no doubt about it -- it would obviously make things very much easier if Cuban forces were withdrawn from Angola."

The British foreign secretary said that after the Sept. 24 meeting, members of the contact group "were united and all pulling in the same direction." He said representatives of the group would tour Africa later this month to brief all concerned parties on details of the New York talks.

Carrington said the group's New York meeting developed proposals for implementing Namibian independence in 1982. He said the group "also agreed on a number of constitutional principles" to be put before a constituent assembly. He said South Africa had dropped its demands for a constitution incorporating minority rights as a precondition for a settlement.

In a speech prepared before Carrington's remarks, Nigerian President Shehu Shagari said that "regrettably, South Africa has felt encouraged by the new U.S. administration intent on linking the so-called issue of Cuban troops in Angola with the question of the independence of Namibia." Shagari added that "this American position is as unjust as it is incomprehensible."