There wasn't any international pressure. There was a withdrawal of the continual and embarrassing support from Britain and the US, the only people other than Israel who hadn't been overly troubled by Apartheid. First from Britain, because as bad as she was, Thatcher was nauseated by Apartheid, then from the US who would have had to actively intervene (as they are right now in a similar context) in order to preserve Apartheid. This was only 25 years after the US had ended its own legal Apartheid.
The US political class was largely indifferent to Apartheid (aside from periodic expressions of mild disapproval of both sides and condemnation of Communist-backed terrorism), so when they saw how the wind was blowing within SA ("fear of a civil war"), and that individual domestic politicians could be damaged or gain politically through their actions towards SA, the US supported the "coup" (as always) so they could keep doing business without interruption.
So I'd instead say popular pressure among citizens of the US and Britain against their own politicians, and the resulting withdrawal of Anglo-American support. Everything else but "international pressure" I agree with totally.