The present exchange rate is extremely favourable (around 12.5 rand to the £pound).
Ask at your bank for the best way of doing it, but if you're offered pay in advance terms only do it: a) whenyou are actually over there and b) when paying by credit card.
Alternatively you could withdraw cash from an ATM before each lesson to cover the cost and nothing more, but check to see how much you'll be charged by your bank.
Check out:
Fool.com: Stock Investing Advice | Stock Research
or
Money Saving Expert: Consumer Revenge - Credit Cards, Shopping, Bank Charges, Cheap Flights and more
Both require a bit of research but they will save you a lot of hassle.