South Sudan (Republic of) has a consulate in London located at Bentinck House 3-8 Bolsover St Fitzrovia London W1W 6AB. Tel: 020 7339 3100
Documents going to South Sudan must be authenitcated first by a Notary Public or other competent authority in the United Kingdom and then have the Apostille applied by the United Kingdom Foreign & Commonwealth Office.
Fees for commercial documents are £50.
Fees for private and educational documents are £20.
Click here for a link to their website.