When did the British first have explorers exploring Africa for raw materials?
Africa has been occupied in the past (though it may have been called "colonization") by many European nations. Portugal seems to have been the first of the European powers to...
...establish settlements, trade posts, permanent fortifications and ports of call along the oceanic coasts of the African continent.
This occurred in the 1400s. The Cape Colony came under British rule, having originally been established by the "Dutch East India Company in 1652." During the French Revolution (when the French forces occupied the Netherlands), the British came to occupy the Cape to keep the French from obtaining this "important strategic location."
As the threat of the French decreased, the Cape was returned to the new government of the Netherlands (the Batavian Republic) in the early 19th Century. However, when Napoleon became a threat, the British once again took control of the Cape to protect it. The Cape would remain under Britain's protection until 1872, when it would become independent, no longer controlled by Britain or the Netherlands.
Early endeavors by two explorers (one Welsh and the other Scottish) became famous for mapping Africa, which had been (most of it) a great mystery to Europeans.
Henry Morton Stanley and David Livingstone would provide invaluable information regarding Southern and Central Africa. (Stanley would ultimately assist King Leopold II of Belgium in colonizing the Congo region for its natural resources.) It would seem, however, that while an ardent missionary in Africa, Livingstone would eventually set his sights on exploration, and this would be for the English:
The British government agreed to fund Livingstone...and he returned to Africa as head of the Zambezi Expedition to examine the natural resources of southeastern Africa and open up the River Zambezi.
This expedition took place in the 1850s and 1860s.