As far as I am aware, once you hold a British passport you are allowed to travel to and from any country where the passport is valid. For entrance to South Africa, the visa is given at the airport on arrival.
However because of recent tension between the two countries, always get the most update to date information prior to traveling.
You will find it much more difficult to get a first time entrance visa at the land borders such as Mozambique though.
If you have any questions, since you are wishing to travel on a British passport to come into SA, rather contact either the British High Commission in Pretoria
or find out what is required before you travel from Home Affairs in the UK.
Hope this helps for future travelers.