Although the SA PPL is valid here in the UK for general aviation, recreational flying, what would be involved in converting it properly into a full UK PPL?
To convert a SA PPL (or any other non-JAA ICAO PPL for that matter) to a UK-issued JAA PPL essentially requires you to do all the ground exams plus the skills test again.
Only when you have more than 100 hours PIC experience are some of the ground exams waived.
Details in LASORS, section C1.2 and C1.3.