Forgive me for being so dumb as to think that if your Student Pilot Licence (SPL) is endorsed with C150 Nil and C152 Nil, then you may fly neither of those aircraft solo? If that is correct then what aircraft is on your student pilot licence for solo privileges?
I don't know that the CAA would update a type rating on an SPL and I certainly wouldn't have thought they'd have done so without some form of paperwork such as a conversion form from your flight school;
There must be those out there with more up to date information as to the modern vagaries of SPLs.
If you are trying to get the South African SPL extended in its range of types so that you can fly a PA28 or a C172 in Scotland then I wouldn't even want to guess at the international ramifications of that.
Surely the best thing for you to do is to telephone your South African flight instructor and if he doesn't know, speak to the CFI, for there must be one of those?