Bishop Abel Muzorewa flew to London today to sign a peace agreement to end black Africa's bloodiest war of independence. Shortly after he took off, the vanguard of a British Commonwealth force to monitor a cease-fire in the guerrilla war began arriving.

Muzorewa left for Friday's signing ceremony with Britain and the Patriotic Front guerrillas saying, "Peace is at hand at last."

Just 45 minutes after Muzorewa's departure, a Royal Air Force jet arrived, carrying more than 100 British troops, most of which will prepare the way for a massive airlift of 1,200 men and equipment to monitor a cease-fire. More than 20,000 persons have been killed in the seven-year-old guerrilla war.

In a sense, the two flights were symbolic of the end of the London peace conference and the beginning of an even more difficult task, making the agreement work on the ground in Rhodesia. Animosities nurtured during the country's 14 years of illegal independence, declared by the white minority to prevent black rule, have left the country an armed camp where there is marked skepticism on all sides about the ability to achieve and maintain a cease-fire.

The agreement, reached after 15 weeks of hard bargaining in London with British Foreign Secretary Lord Carrington playing the broker between the two warring sides, calls for elections within three months to choose a black majority government to rule on the basis of a constitution negotiated at the talks.

Britain has taken over nominal administration of the country during the interim period with its governor, Lord Soames, ruling by decree.

Carrington's succesful method at the conference involved pushing off the most difficult problems to the next phase of the four-stage talks. Now, in effect, Soames will inherit the key remaining problems -- basically how to get the country through the delicate period leading up to election and installation of a new government.

The most immediate problem will be establishing and enforcing the cease-fire agreement to be signed Friday in London by Carrington, Muzorewa, and Patriotic Front coleaders Robert Mugabe and Joshua Mkomo, who are to sign in alphabetical order.

Over the next week, 1,200 troops to monitor the cease-fire are scheduled to arrive from Britain, Australia, New Zealand, Kenya and Fiji. They are to be deployed in the countryside by Dec. 28, the day the truce nominally begins, to observe the movement of Rhodesian security forces back to 40 bases and the assemblying of the guerrillas at 16 sites.

The 100 British troops who arrived today made up the first contingent in an airlift that will involve about 12 planes a day arriving for the next week. Britain will use about a dozen of its own C130's and VC10hs and rent two U.S. Air Force C5A Galaxies and five C 141 Starlifters to move in the troops and equipment.

Just before the first airlift plane arrived, an Air Cargo Oman DC8 jet took off on a flight familiar to Rhodesians. The cargo line was one of the main carriers used to break economic sanctions imposed by the United States against Rhodesia 13 years ago. Britain and the United States have now lifted those sanctions.

The London agreement calls for the cease-fire to take force Jan. 4 and from then on any troops that have not reported to their bases from either side will be regarded as illegal.

The process of enforcement of the cease-fire is mainly based on good will and trust. The thinly spread Commonwealth forces, which will monitor the activities of about 50,000 or more troops, as of now still at war, will not function in a peacekeeping role. Instead, they will report violations to a British-run cease-fire commission that will call on the offending force first to put its house in order.

If that fails, Soames will have the power to "use the forces at his disposa" to end the violations, meaning that he could command one side to put down the violations of the other side.

The British openly admit that the cease-fire will work only if the participating parties want it to, but the hope is that with an election in the offing by late February or early March it will be in their interests to maintain the peace.

The first few days after the Jan. 4 date for the truce to become effective could well determine the outcome of this ninth major endeavor to end the Rhodesian crisis.

So far Britain's recolonization of Rhodesia to quickly decolonize and produce an independent Zimbabwe -- the African name for the country -- has been a low-key affair with Soames taking few actions as he waited for signing of the London agreement.

This has tended to assuage the white minority and the Muzorewa government that disbanded to make way for the British administration, although Soames' failure to lift the political ban on the Front has annoyed the guerrillas. He is expected to end the ban Friday.

Soames is relying on the existing Rhodesian civil service to carry out the day-to-day business of the government with just a thin overlay of British officials. Whether Mugabe and Nkomo will be content with such a situation will be tested after they return, probably sometimes next month.

A British soudce, giving an upbeat outlook about the prospect for fair elections, pointed out that there was little optimism when the London talks started 101 days ago. "We've wittnessed one small miracle. Let's hope for another," he said.

On the more pessimistic side, however, one cynic noted that perhaps the airlift, which so far had not been given an official name, should be called "Operation Fingers Crossed."