The green and white flag of a black African giant, flapping lazily over the broad expanse of Whitehall, signaled new African pressure on the West against apartheid and South Africa's stand on Namibia.
The giant was Nigeria, the richest and one of the most influential black African countries. The flag flew for the state visit here of Nigerian President Shehu Shagari, who brought a retinue of about two dozen ministers, advisers, and businessmen.
Discretely but pointedly, the poet-turned- president asked Prime Minister Margaret Thatcher to study again the case black Africa makes for sharp economic sanctions against South Africa designed to end apartheid and give independence to Namibia.
Together with other black African leaders, President Shagari fears that President Reagan might be less stern with Pretoria than he thinks Mr. Reagan ought to be. Black Africa is anxious to stop Britain from following the same path.
Sources close to the talks with Mrs. Thatcher and Foreign Secretary Lord Carrington March 18 said President Shagari appeared to see the threat of sanctions a useful tool agianst South Africa. He emphasized that the West stood to gain far more from long-term ties with Nigeria and the rest of black African than it did from short-term links to South Africa.
British officials reply that sanctions failed against Rhodesia (Zimbabwe) and have even less chance against South Africa.
But Britain does not want to remove the threat entirely. It leaves open the door to eventual United Nations Security Council action -- while urging Nigeria and others to give Mr. Reagan more time to work out a coherent policy on Namibia.
Nigeria is using massive income from oil to pump more than L65 billion ($146. 9 billion) into its economy over the next five years. So it also represents a great commercial opportunity for hard-pressed Britain.
The Shagari visit marks the end of a period of tension between London and Lagos over Rhodesia independence and over oil. Nigeria nationalized British Petroleum holdings in August 1979, but is now to pay L60 million ($1.35 billion) in compensation.
President Shagari reminded London that Nigeria was Britain's largest market in Africa for British products and services. Nigeria is also the key African market for European Community and the United States.
Britain is anxiously to sell more but it is having trouble finding Nigerian products to buy, since its own North Sea oil is the same grade as Nigerian oil.
President Shagari objected to anew rule here that all overseas students must pay full fees at British universities. Lord Carrington said he regret ted it, but that Britain had to economize.