Britain said today it will move this week to put into effect the independence constitution for Zimbabwe-Rhodesia and appoint a British governor to supervise new elections, there, whether or not Patriotic Front guerillas agree to a ceasefire.

Lord Carrington, Britain's foreign minister and chairman of the Rhodesian peace talks here, made this surprise announcement tonight in a showdown attempt to force Patriotic Front leaders Robert Mugabe and Joshua Nkomo to join in a final settlement by Friday.

"We shall be taking the action which is necessary to enable us to put a settlement into effect," Carrington said at one of his rare, calculated appearances before reporters at the conference.

"I do not despair of reaching an agreement . . ." Carrington said, "but we simply cannot wait forever for the Patriotic Front's reply."

British Cabinet officials tonight approved the final 84-page draft of the constitution, which is to be given to the peace conference delegations tomorrow and enacted by the British government later this week. They also created the position of interim British governor, which could be filled as soon as next week and the person sent to the breakaway British colony immediately.

Both the Front leaders and the present Salisbury government of Prime Minister Abel Muzorewa have agreed on the substance of the constitution and the arrangement for implementing it and holding new elections under the British governor.

The Salisbury government also has agreed to the British cease-fire plan involving the supervision of a monitoring force of more than 1,000 British and British Commonwealth troops. But Mugabe and Nkomo have not yet accepted it, contending that it does not adequately assure the safety of their forces. They are also believed to be stalling while more of their troops and supporters infiltrate into Zimbabwe-Rhodesia.

Front spokesmen said tonight that Mugabe and Nkomo would answer Carrington at a press conference tomorrow.

While the British have not named the governor, he is likely to be, in the case of an all-parties peace settlement, 59-year-old Lord Soames, the son-in-law of Winston Churchill and present leader of the House of Lords.