If you visitors are primarily in Zimbabwe, hosting in Zimbabwe is the best. The next best is whatever closest country to Zimbabwe there is, but only if there's a direct internet link between those countries, and Zimbabwe and SA have direct internet links.
Ultimately, the lesser internet hops (Different stops that you requests and files make before they reach a site and back to the user's phone (or computer)) the traffic makes, the faster the speed. UK is therefore slower than South Africa, but it is generally faster than the US as most of our traffic in Zimbabwe goes through Europe before it reaches the US.