oh-o.... lots of:
(a) accurate information that is being misunderstood
and
(b) inaccurate information that sounds believable
NorthSouth is quite correct that if you go and take lessons but do not get a licence (ie: you do some proportion of the foreign licence course but not finish and actually get the licence) and then arrive back in the UK, you can only claim max 10 hours credit towards a JAA PPL course. This is unfortunately being misread or misunderstood by some, but NorthSouth is correct and accurate as wha tis written concerns someone just taking training abroad and not getting a foreign licence.
On the other hand, if you come back with a SA PPL, it is automatically validated for private use in this country without formality. You will therefore be legal to fly here, but hopefully you will take a few lessons on navigation around the UK or team up with local flyers to become accustomed to it. Rental checkouts will be required just like for everyone else. You NEED never convert to a JAA PPL. (Many don't, and others just fly here for a year or two on the SA PPL until they finally end up doing a JAA CPL course and never actually convert at PPL level)
If you WANT to convert to a JAA PPL from a valid SA PPL, you always need a JAA medical and you always need to pass the full initial PPL Skills Test (the big one, 2.25 hours approx including nav and general handling), no matter how many hours you have. The only difference your hours make is to decide whether you have to do ALL JAA ground exams (under 100 hours experience) or only two of them (Air Law and Human performance). Only those 2 are required in the ground exam department if you have 100 hours total or more, but you STILL need the flying test, medical etc. Your foreign PPL needs to be current at the time of conversion to JAA too whenever you apply.