Strange I had colleagues prior to specialisation work in NZ and Canada. US prefers pre specialisation doctors because they prefer their own programs, you have to write 3 exams for the US.
US goes on the university where you graduated as I think does everyone else. SA doctors with GMC registration can work in the UK and can also enter further programs there. There is a separate registration for Ireland where many Saffers work too because of the language and ease of entry.
This has nothing to do with HPCSA or local SA licensing.
That's for clinical jobs. For non-clinical jobs there are fewer barriers to entry.
There's a shortage of doctors in the UK at the moment and they take people easily. If you don't have GMC registration, you have to write exams, if you already have GMC it's pretty simple.