All stations...
you can do it at London Gatwick, Aviation Medical Centre.
Can fish out details for you if you want.
There should be a list of foreign AME's approved to do SA medicals, on the caa website
www.caa.co.za
Also, SA CAA can accept a foreign class 1 medical in order to revalidate your sa one, best speak to them as I don't have the exact reference.
Cheers, mooi bly