The Electoral Commission of South Africa has confirmed that SA citizens voting abroad will cast their ballots on April 27th, 2019.
In order to be eligible to vote abroad, if you have registered to vote you will still need to complete a VEC 10 form indicating where you intend to vote. For South African citizens in the UK, that will be the South African High Commission in Trafalgar Square, London. See map.
VEC 10 forms can be completed online, and must be submitted before midnight (UTC + 02:00) on 13th March 2019. That’s 10pm in the UK.