You would need to apply for a UK visitor visa. To enter England, Northern Ireland, Scotland or Wales (which make up the United Kingdom), you would require a UK visitor visa before leaving South Africa.
Your visa will normally be valid for up to 6 months. This allows you multiple entries into the UK during this period.
You may however apply for visas that carry a longer validity period. The validity periods range for up to 2 years, up to 5 years and up to 10 years but you may only stay for a maximum of 6 months per 12 month cycle.
The longer the validity of your UK visitor visa, the higher the UK consular fee will be.
Yes, all visa applicants would need to go in person when submitting their UK visa application as their finger prints and photos (i.e. biometrics) would need to be taken during their visa submission appointment. Children of all ages would also need to appear in person at their appointment.
According to the UK Border Agency itself, these are the rules. South Africa is part of the Direct Airside Transit Visa (DATV) agreement:
If you are a national of a country covered by the DATV system, you may be able to transit the UK airside without a visa. To qualify for this exemption;
Additionally, you must hold;