Does anyone know what criteria our government uses when designating a group as a terrorist organisation?

I keep thinking about Nelson Mandela and the ANC, which the UK and the US proscribed as a terrorist organisation for its violent resistance to the apartheid South African government, and comparisons to Hamas.

The UK supported and were allies of the government of South Africa - until public opinion and moral outrage and the freeing of Mandela to become SA president, forced them to ‘switch sides’. Until that point, we were clearly supporting the aggressor in (as far as I can see) the same way we are now supporting the massacre of Palestinians by Israel