I wouldn’t wish for one moment to question the work of American missionaries, but during the great period of Imperial expansion in the 19th century their numbers in Africa were still small compared with the legions sent out by the Church Missionary Society, the Baptist Missionary Society, the Methodist Missionary Society and others. It’s no accident that today there are more active Anglicans in Nigeria than in the UK itself.
Africa at the close of WWI was more than 80% pagan, and the largest number of those who weren’t were Muslim, and the majority of the small Christian minority were Catholic.
The chief effect of the UK Christianization was the establishment of the Church of England as the vehicle through which other missionaries — many bearing no affiliation to the Church of England — did their work.