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After entering, you would not be able to change your status from the visitor to the employment category from within the country. While South African nationals may be allowed to remain in the UK for a maximum 6 months, visa conditions impose strict employment restrictions.
You can only work in the UK with the proper authorization and require a certificate from a licensed sponsor before you can apply to work, whether it be for compensation to do unpaid voluntary work for a charity.
Fees are not refunded when a visa is refused.