Foreign Secretary David Owen insisted yesterday that Anglo-American diplomacy was making "some progress" toward peace talks among warring blacks and whites in Rhodesia.

Owen did not elaborate before a grim House of Commons, assembled to hear him condemn the Friday night slayings of eight British missionaries and four of their children.

Diplomatic sources here agreed that there now is an increasing prospect that white Prime Minister Ian Smith would meet the black nationalist leaders, Robert Mugabe and Joshua Nkomo. Even if the meeting does come off, however, officials here are increasingly skeptical about its prospects of producing a peace.

The "progress" Owen mentioned is said to center on a change of heart by Smith. He has publicly expressed his unhappiness with the "internal settlement" he has worked out with black politicians inside Rhodesia, providing heavily qualified majority rule.

In the words of one diplomat here, that settlement "is very loose ground." The appeal by Smith and his black colleagues, Bishop Abel Muzorewa and the Rev. Ndaganingi Sithole, urging individual guerrillas to lay down their arms, has produced only a meager result.

The black members of Smith's team are thought to be losing popularity with their followers. The internal settlement has not altered the Rhodesian practice of reserving the best land and the best jobs for whites. Diplomats believe Muzorewa and Sithole would go along with the talks if Smith agreed.

On the guerrilla side, both Mugabe and Nkomo are publicly committed to talks. As Smith moves toward them, however, they can be expected to regard this as a sign of weakness and intensify their demands. This is why both U.S. and British diplomats here are slightly more optimistic about talks but increasingly pessimistic about the outcome. The best informed diplomats here have always expected a bloody resolution of the Rhodesian struggle.

Owen described Friday night's atrocity as an "appalling tragedy" that "confirms the urgent need" for peace talks. But opposition Tories, expecting an election here in October, suggested that the Labor government was partly to blame.

John Davies, the Conservative foreign affairs spokesman, accused the government of "cold shouldering" the internal settlement and thereby encouraging those who seek power by the bayonet, the club and the gun."

Earlier, an extreme right-wing Tory, Patrick Wall, charged Owen and Andrew Young, the U.S. ambassador to the United Nations, of having "a share of responsibility."

Owen replied that the Anglo-American position is one neither of encouragement nor discouragement for the internal deal. He repeated his view that no settlement would bring peace, however, unless it included the guerrilla leaders.