Lord Soames, Winston Churchill's son-in-law and postwar aide and formerly ambassador to France, today was appointed British governor in Zimbabwe-Rhodesia. A blue-ribbon team of British officials was named to assist him.

The announcement of their appointments and a public reminder from the British foreign secretary, Lord Carrington, that Soames would be in Salisbury "in the course of next week" were designed to speed the signing of a formal cease-fire agreement ending the seven-year guerrilla war there.

But negotiations about implementation of the cease-fire bogged down at the Rhodesian peace conference here. Patriotic Front guerrilla leaders continued to object to having their forces herded into 15 "assembly places" they called "detention camps" while those of the Salisbury government of Prime Minister Abel Muzorewa operate from more numerous bases and have "the run of the country."

Neither the Patriotic Front nor the Muzorewa government would even state the number or location of their forces at today's conference session, chaired by Sir Anthony Duff, a British diplomat who will be deputy governor to Soames in Salisbury. They finally agreed to give the information in sealed envelopes to Duff or Carrington, who would keep it confidential.

"We've gone rather slow so far," Carrington acknowledged today about the final "trying up of details" phase of the peace talks, which he had hoped would take only a few days.

"There is a sense of urgency about this," he said at a press conference with Soames, Duff and other British officials bound for Salisbury. "Over the weekend, we're going to be working hard."

Soames and the others will go to Salisbury next week whether or not the peace agreement has been signed by then, Carrington explained, because "preparations have to be made at the other end."

A Patriotic Front spokesman warned today that Soames might be in danger under those circumstances because "anyone who loiters around in our operational areas is fair game."

But Soames said he was not afraid. "I'm not going there defiantly," he said. "I'm going there to help. I go into this with considerable confidence, but I don't minimize the difficulties."

The arrival of a British governor in Salisbury would appear to meet a condition set by the Thatcher government for the lifting of economic sanctions against Zimbabwe-Rhodesia. Britain has said it would lift the sanctions once Salisbury "returned to legality" under the transition plan worked out in the London conference.

The United States has not lifted its sanctions against the Salisbury government yet, but the Senate voted 90-0 Thursday for a resolution calling on President Carter to lift U.S. sanctions as soon as the British resume control of the breakaway colony, or by Jan. 31.

Soames, 59, is a plump, jowly-faced man with a well-known love of good food and wine who was made a lord for his long, distinguished public service. He married Churchill's youngest daughter, Mary, and he was believed to have been Britain's virtual leader for a time in 1953 when Churchill had a stroke. Young Christopher Soames was Churchill's parliamentary secretary and closest aide.

His favorite period in public life was as ambassador to France from 1968 to 1972 when he helped lay the groundwork for Britain's often-delayed entry into the Common Market in 1973. He suffered heart trouble in the mid-1970s, recovering from major heart surgery in 1977.

When the Conservatives regained power in Britain this year, Prime Minister Margaret Thatcher made Soames Lord President of Council in charge of the civil service. Only yesterday he announced a controversial reduction of 40,000 government jobs as part of Thatcher's continuing public spending cuts.

Soames acknowledged today that he has never been to Rhodesia and he seemed unprepared for detailed questions about the situation there or the peace talks here. "I've only been appointed today," he said, turning to Carrington for assistance.

Duff will be Soames' top aide in Salisbury during the interim British rule that will return the breakaway colony to legal independence under a black majority government after new, British-supervised elections.

Sir John Boynton, a longtime local government official, will be the election commissioner, heading a group of 100 British election officials who will supervise the Salisbury government's civil service to ensure that the election is conducted fairly.

In addition, a force of 1,200 or more British and Commonwealth troops will monitor the opposing military forces to ensure that they observe the cease-fire.