Hi south african PPL is valid in the uk no need to validate in the uk.The best school i would advice you to go to is 43 air school,beyond no doubt it is in the top five flying schools in the world,i did my PPL and COMM lats year,exellent standards of aircrafts instructors,many UK airlines are sending their cadets there too,you can check them out on
WWW.43airschool.com you will never regret The PPL should take at least 8 weeks
BEST OF LUCK