Foreign Minister Pik Botha today in a statement to a group of government supporters in Durban restated that South Africa was "disappointed" by the British government's move to sponsor an all-party conference next month in a search of a peach settlement for its former colony of Zimbabwe-Rhodesia.

In private, however, there is a "guarded feeling of optimism" in the Foreign Affairs Department that a "democratic" solution can come out of the London conference, according to one department official.

South Africa's opposition to the British initiative stems from what the government perceives as Prime Minister Margaret Thatcher's "turnabout" at the Commonwealth conference in Lusaka, Zambia. There, Thatcher abandoned her apparent intention to lift economic sanctions and recognize the moderate Zimbabwe-Rhodesian government of Bishop Abel Muzorewa without attempting to bring his socialist-inclined guerrilla opponents into the government.

Pretoria preferred that Britain recognize Muzorewa since that fit with South Africa's regional strategy of building up neighboring, non-Marxist, economically independent buffer states. Political observers here said that in recent months the government of Prime Minister Pieter W. Botha has made a definite commitment to maintain economic and political stability in Zimbabwe-Rhodesia and the territory of Namibia (South West Africa).

A central assumption in this thinking is the continued presence of the white community in both countries, regarded by South Africa as a necessary ingredient for stability. This in turn requires non-Marxist or non-socialist governments, according to the government, since the majority of whites, it is assumed, would depart if the present capitalist economies are disrupted or altered drastically.

This is why, although South Africa is prepared to be a "good neighbor" with the Marxist government of Samora Machel in Mozambique, it does not appear willing to permit Marxist governments in Zimbabwe-Rhodesia and Namibia.

The London conference seeks to get all parties in the Zimbabwe-Rhodesia conflict, including Soviet-backed Joshua Nkomo and avowedly Marxist Robert Mugabe, to agree to plans for new elections. Such a solution threatens South Africa's regional strategy because it raises the prospect that a government will be set up with the participation of -- if not under the control of -- the socialist forces.

South Africa's guarded optimism about the London talks is based partly on the belief, perhaps a miscalculated one, that the guerrilla leaders will not participate in a settlement if the Thatcher government persists in what South Africans call a "democratic" solution.

Meanwhile, South African officials and British envoy Sir James Murray are engaged in talks here aimed at breaking the impasse in negotiations to implement a U.N. independence plan in Namibia. Murray, who reportedly heard complaints from Prime Minister Botha about the British change on Zimbabwe-Rhodesia, is hoping to leave here with a firm commitment from South Africa to go ahead with the plan, according to Western diplomats.

The main proposal under discussion is the creation of a 60-mile-wide demilitarized zone along the Namibian border with Angola -- a major concession from Angola, which consistently had refused any suggestion of U.N. troops on its soil. The buffer area is meant to allay South African concern about infiltration of insurgents of the South West Africa People's Organization from their Angolan-based camps during a cease-fire.

Although talks appear to be going well, South African officials publicly are throwing cold water on the demilitarized zone proposal.