I can't remember the exact amount, but you are generally allowed to bring in up to R3000 (open to correction) without paying duties. Anything in excess of this you generally have to pay local VAT and a fixed admin fee (don't think it's too much). So 14% at what ever exchange rate SARS are using that day.
As mentioned you may be able to claim back the VAT on the UK side. As you leave the country they will pay you the VAT you've already paid on the item. Make sure you have the receipts, for the South African Customs too.