How did you get the annual pass if you're in SA? Did you register as a UK citizen?
Blizzard did open it up for a couple of countries a couple of weeks after the annual pass started. I think South Africa was included in that.
If you are having trouble registering for annual pass, then you need to contact Blizz. There are a number of things that can mark an account as invalid, like parental controls. They can check why your account can't register and what you need to do to register.
Its not possible to change your country anymore, by simply clicking a button. Now you have to contact customer support in order to do that.