The UK, along with many other western countries are making it increasingly difficult for foreigners to visit, especially those travelling from poorer nations such as those in Africa. And visas for the likes of Zimbabweans just to visit have to apply long in advance and cost several hundred US Dollars.
So many African countries are now imposing Visas on western visitors travelling to their countries. Mostly these will be paid for as you enter the country and can be quite expensive, especially if you are paying for a family to visit.