Do I qualify for British Citizenship through double descent?

I would like to get my BRITISH CITIZENSHIP by double descent. I DO NOT want to apply for an ancestral visa or a work permit, unless I cannot get British citizenship.

This is basically the my situation:

My UK line of descent is:
(My UK Grandmother) –> (My South African Father) –> (My South African self)

I have these documents:
1. Certified copy of my grandmother’s entry of birth.
2. My father’s unabridged birth certificate.
3. My own unabridged birth certificate.
4. My South African passport.

I live in South Africa. I have no criminal convictions, I am single and divorced.

Let me reiterate, I am NOT wanting an ANCESTRAL VISA or a work permit, I am wanting to get my BRITISH CITIZENSHIP.

So I need to have references to the UK law which show that I qualify
– I need to know what I need to do to get citizenship
– I need to know what documents I need exactly
– and anything else I need to know

I don’t want long stories, just the facts and instructions that pertain to my situation.