Every flying hour is loggable and valid in any country and under any licence, so the all the hours you build will count!!.
In this respect doing the CPL/IR in S.A. will give valuable CHEAP instrument flying. But you'll never use the licence, it's only usefull because you're now familar with instrument flying. That kind of flying here will cost you an arm and leg.
The only snag, whatever you do and even if you have the CPL/IR you'll still have to 55hours of IF/IR in the UK before the UK CAA give you an ATPL JAA licence (thats assuming you've got all the exams and pass the ATPL flight test).
At the end of the day, only do your flying upto CPL/IR in S.A.
DO NOT DO the CPL/IR flight test. The reason, you can't do the flight test without the exams. Which means ground school for 6 weeks or more in the classroom. Not worth it.
Come back to the UK and do the ATPL ground school here, get the ground exams, then get your flying done.
SA ground school will get you familar with a lot of the stuff in the JAA, but now your doing 2 sets of exams, 2 sets of ground schools, 2 sets of DOSH!!!!.
By the way, this is all my opinion.
Enjoy.