Soviet leader Mikhail Gor-bachev's visit to Cuba and Britain beginning Sunday could not be better timed for keeping the momentum for change going in southern Africa.

By reducing his troop strength in Angola, Cuban President Fidel Castro has been intimately involved in the settlement calling for indepen-UFanalysisdence for South African-ruled Namibia, where a U.N. monitoring group began its first official day of operation Saturday.Cuba receives $5 billion in annual aid and subsidies from the Soviet Union, which could apply pressure to Cuba to make sure it keeps its part in the peace settlement reached last year setting out a timetable for Namibian independence.

South Africa rejected repeated U.N. demands to let go of Namibia until Cuba - with Soviet prodding - agreed to withdraw its 50,000 troops from Marxist-ruled Angola.

After the April 2-5 visit to Cuba, British Prime Minister Margaret Thatcher, who will meet Gor-bachev April 5-7, will be back from a major tour of the African subcontinent where countless thousands have died in war and famine since the Portuguese withdrawal from Mozambique and Angola in the mid-1970s.

British and Soviet officials have said white-ruled South Africa will figure high on the agenda.

"The Soviet Union has now taken a less aggressive stance toward South Africa by toning down its rhetoric and calling for a peaceful settlement of apartheid discrimination against blacks," said a Western diplomat.

"The trouble is they know next to nothing about the place and they are trying to learn all they can from Britain, and the foreign office has helped Moscow establish contact with South African intellectuals," he said.

The United States, which helped set up the Namibia settlement that saw United Nations monitors move into the vast semidesert territory twice the size of California, now appears well in the background as Britain and the Soviet Union take the diplomatic initiative, the diplomat said.

Aside from the progress toward independence of Africa's last colony, London also views the timing as important because it plans to raise sticky human rights issues with Gorbachev.

British officials said they are anxious to learn all they can about recent Soviet parliamentary elections because they feel the internal situation in the Soviet Union has a direct bearing on Western security.

"What we are witnessing now is better than the old-fashioned Soviet diplomacy when they thought they had to export their system," a NATO diplomat said.