If that's true, then I'd have to change what I said about the UK's response: no matter what's distracting Callaghan at home and no matter how much Thatcher wants an African anti-communist mate, neither of them would be able to turn a blind eye to an invasion that threatened UK contractors. They'd be telling South Africa to cut that out and they better damn well not hurt one of Her Majesty's subjects when they pile in.
South Africa could very easily stage a "brutal terrorist attack" on Rhodies and UK contractors, and then claim that Rhodesia was negotiating with the kinds of terrorists responsible for the violence, and then "step-in" to "mediate"?
It could even make for an interesting turn later on in this TL! A couple years on, evidence emerges that the terrorist incident which sparked the SA intervention was staged, and the UK and US immediately withdraw their support.