Britain today rejected efforts by the Patriotic Front alliance to make further changes in a proposed independence constitution for Zimbabwe-Rhodesia and set thursday as a deadline for the guerrilla group to accept the British draft.

Front spokesmen said that their response at the British-sponsored conference, now in its fifth week, would not be a flat yes or no and added that they would not let British Foreign Secretary Lord Carrington "dictate" to them.

Despite the brinksmanship being employed by the british to force progress to settle the 14-year-old Rhodesia issue, there were hesitant signs that Britain and the guerrillas would find a diplomatic way to avoid a confrontation.

Patriotic Front coleader Robert Mugabe emphasized that substantial progress has been made between the two sides in narrowing their differences, although there were "some problems left." The accent in his speech at a correspondents' luncheon was on the positive side despite the Thursday deadline.

British sources also expressed cautious optimism that a way would be found for the Front to give conditional acceptance to the constitution, thus allowing the conference to go on to the more difficult issues of interim arrangements to bring about an independent black-majority government.

"What we need is a 'yes,' even if it is a 'yes, but,'" one British source said.

The rival biracial Zimbabwe-Rhodesian government has already given its conditional agreement, although white former prime minister Ian Smith is opposed.

The Patriotic Front objections have been narrowed to four key provisions: protection of land rights for the white minority, continued citizenship for those who came to the country since the illegal declaration of independence in 1965, pensions for officials of the breakaway government and entrenchment in the constitution of some guarantees for whites.

Carrington told the Front at today's meeting at Lancaster House that the outstanding issues "simply cannot be left on the side." adding that "there has been ample time and occasion for the discussion of all issues."

Praising the Front for its concessions, he nevertheless said the British draft "is the only basis on which it is not possible to reach full agreement'

"We cannot now reopen these matters to meet the views of the Patriotic Front any further than we have gone to meet them already. Nor can we proceed with discussion of the pre-independence arrangements while either side reserves the right to reopen" talks on the constitution.

He then called for a decision by Thursday when he returns from addressing the Conservative Party conference at Blackpool where he faces some right-wing opposition to his negotiating policy.

Patriotic Front spokesman Eddison Zwogbo appeared to go some distance toward Carrington's opposition to reopening the constitutional issues, saying there would be no need to return to unsettled problems in agreement could be reached on the overall package, including the transitional arrangements.

Zvogbo twice referred to "acceptance" of the constitution in the context of linkage with the transitional arrangements that involve the key question of what to do with the warring military forces.

This formulation could provide a way around the looming confrontation Thursday by allowing the Front to conditionaly accept the constitution with reservations on certain issues. These would not have to be discussed again but simply be weighted in terms of an overall agreement.

Mugabe added to the positive atmosphere, saying proposed changes by the Front yesterday amount to "agreement to a substantial portion of [Carrington's] constitution and we only have reservations on a few areas."

As usual in the long, tortured history of Rhodesia negotiations to obtain black-majority rule, however, a mixed image emerged.

Mugabe complained about Carrington's negotiating methods and then did some almost obligatory saber-rattling. If the trend continued and it appeared that the talks were heading nowhere, he said, he would send his military leaders here for the conference back to the battlefield to escalate the seven-year-old war.

"We can win without Lancaster," he said, referring to the talks' locale, and added that the Front wanted a negotiated solution "but otherwise we can achieve peace through the barrel of a gun."