If that figure of 7.05 GBP per megabyte is correct it would be a much better idea just to purchase a prepaid SIM in South Africa from any MTN cell shop, supermarket or convenience store. http://www.o2international.co.uk/standardrate_and_data.aspx
If you purchase a bundle from MTN online the cost per megabyte is even cheaper (standard rate is R2 per megabyte). If you visit one of the MTN shops in SA they should be able to assist you to activate the SIM and help you to load one of the bundles shown above if you think that you might have trouble. You would need to check that your phone or datacard is unlocked before arriving in SA i.e. can you use your current datacard on any of the other UK networks like Vodafone or 3 just by switching the SIM?
If it is locked you may be able to get the unlocking code free of charge from the network provider (O2). Otherwise the simplest way to unlock it would be to investigate the service at www.kulankendi.com. This company can unlock most of the current datacards over the internet without you needing to physically mail it to them.