If you're a citizen of the EEA (European Economic Area) nations or Switzerland, you don't need a visa to enter or work in Britain – you can enter using your national identity card.
Visa regulations are always subject to change, which is especially likely after Britain's exit from the EU on 29 March 2019, so it's essential to check before leaving home.
Currently, if you're a citizen of Australia, Canada, New Zealand, Japan, Israel, the US and several other countries, you can stay for up to six months (no visa required) but are not allowed to work.
Nationals of many countries, including South Africa, will need to obtain a visa: for more info, see www.gov.uk/browse/visas-immigration.
The Youth Mobility Scheme, for Australian, Canadian, Japanese, Hong Kong, Monégasque, New Zealand, South Korean and Taiwanese citizens aged 18 to 31, allows working visits of up to two years but must be applied for in advance.
Commonwealth citizens with a UK-born parent may be eligible for a Certificate of Entitlement to the Right of Abode, which entitles them to live and work in the UK.
Commonwealth citizens with a UK-born grandparent could qualify for a UK Ancestry Employment Certificate, allowing them to work full time for up to five years in the UK.
British immigration authorities have always been tough; dress neatly and carry proof that you have sufficient funds with which to support yourself. A credit card and/or an onward ticket will help.