Here in South Africa there are no Theravada centres or temples or any presence that I know of.
http://www.peaceforafrica.org/the-temple/
I took refuge vows in the Tibetan Nyingma tradition and also received a few tantric initiations however I have become just as weary of the theistic type worship and promises of help and blessings from deities and bodhisattvas as I did of god, angels and saints intercession when I was still a Christian.
I'm not sure what you mean by "theistic type worship". There is no creator god in any school of Buddhism. Deity practice is much more profound than "theistic type worship". The bigger issue here is why you would make vows and take on responsibilities, only to then "become weary" of the fundamentals of what you took on. Tantric deities (in the Vajrayana) and the existence of bodhisattvas (in the Mahayana) are fundamental to the Nyingma worldview. It seems to me you don't like aspects of your previous faith. Well, that's understandable, but it's not a good basis for making important decisions. If it were, any time you encounter anything that smacks of your old tradition you would develop strong aversion to even beneficial things. And I'll just tell you upfront: there are some texts in the Pali Canon that portray the Buddha as having great supernatural powers. I can't guarantee everything you find in Theravada will conform to a materialistic, scientific worldview. Please reflect on how the Buddha's teachings could apply to your previous tradition. Is your attitude toward it based on harmlessness, non-ill will, right speech, compassion? More than anything, I think you'd really benefit from that.
if there’s no Sangha in my home country, is practice in the Theravada tradition something which can be done alone?
Yes, it can be done alone. But the Buddha encouraged the development of virtue in community before going off alone. So what you can do for now is learn about the Theravada path, as much as you can. If you can find a guide (monk or experienced lay teacher), that would help a lot. Keep in mind that both in terms of meditation and in terms of doctrine you can make mistakes that add up over time to obstacles on the path, so it's best to have spiritual friends to check your efforts.