Pro 1966,
I'm assuming you mean converting your SA PPL to a UK JAA PPL? As you are studying towards your Commercial Licence there is no need. When you finish your ATPL's at BGS - hour build, then start the CPL course with your SA PPL. The only requirement is that you have a valid ICAO PPL. On completion of the course and on reaching the 200hrs necessary for licence issue, you can submit to the CAA your logbook, paperwork, skills test, hefty fee and SA PPL etc - then get your UK JAA CPL issued without converting to JAA PPL at all.
Not the most interesting post on here today I'm sure - but I hope it answers your question. Also have a look at Irv Lees' website: higherplane.flyer.co.uk
CD.