Export from UK and into SA is no major issue...FAC and letter of invite will do it!
However...If flying within SA they ask that any ammo is removed from hold luggage and goes "securely" with but separately from your firearm to point of destination...ie two cases...rifle & ammo. Can be a hassle if you haven't got a spare Pelicase! If you are checked straight thro' shouldnt be a problem!
The other thing to be aware of in Africa generally is that EVERYONE inc Customs officials will ask you for a bribe...sorry did I say bribe...I meant "tip" in order to expedite your firearms through their hands. If going thro Jo'Burg you will be pounced upon by "helpful" porters who claim they will whisk you thro' customs etc etc for a tidy fee.....Be firm and polite and mainly pretend you've only just arrived and have no cash and just assertively and confidently work your way through!