Firstly, if you do not get a South African CPL (or any other ICAO CPL) you will have more to contend with in the UK. The reason being you will have to undertake a full CPL modular course back in the UK instead of "training as required", and you will pay about another £150-200 for your medical. So although you will save yourself some study time in South Africa, you will end up paying considerably more.
Secondly, if you do have an ICAO CPL and wish to convert that back to a UK JAA licence (cheaper than simply having hours but no licence) take a look
here for what costs you can expect. That
has to be budgeting into the equation when deciding on the cheapest route (which is not always best; experience and variety are important too).
If you do train abroad, make sure you have completed all the requirements linked in that other thread, particularly night flying if you want that on your UK licence - night flying in the UK can be an expensive pain to arrange.