Where does the white missionary go when he leaves Mbanta?

When Mr Brown leaves Mbanta, he goes home to England due to his poor health. His departure occurred after a period of flourishing for his church:

Mr. Brown's mission grew from strength to strength, and because of its links with the new administration it earned a new social prestige. But Mr. Brown himself was breaking down in health. At first he ignored the warning signs. But in the end he had to leave his flock, sad and broken.

We are told that Mr. Brown went home, which, in his case, was England. He leaves behind him a transformed Mbanta, in which "the new religion and government and the trading stores were very much in the people's minds and eyes."
The significance of Mr. Brown's departure in the narrative is that it signals the beginning of the end for the church in Mbanta. Because Mr. Brown had served as a moderating influence, he had been able to grow his church while establishing basically respectful relationships with his neighbors; but once he leaves, things take an unfortunate turn, as Mr. Brown's replacement, Mr. Smith, has a different manner and style of church governance and community relations. Whereas Mr. Brown aimed for reasonable compromise and some measure of tolerance, Mr. Smith is much more extreme.
Thus, the departure of Mr. Brown and the sensibility that he represented makes the survival of the church in Mbanta impossible. Only Mr. Brown's leadership made the difficult situation even minimally functional; in the presence of Mr. Smith, relationships rapidly break down and the church is destroyed.