Registration as BC for Kids Born Overseas - Supporting Docs

Hi

I need some help with the documents required when use the section 3(2) path for my 3 kids born in South Africa.

I have British Citizenship by Decent but have lived in the UK for 3 years prior to the birth of my children. Both of my parents (kids Grandparents were born and brought up in UK and are British Citizens in their own right.

Documents I am required to submit
1. My Zimbabwean birth certificate - I have a photocopy
2. My parents UK birth certificates - I have a photocopy
3. My parents UK marriage certificate - I have a photocopy

Are photocopies sufficient? Do they need to be certified by a commissioner of oaths?
If copies are not allowed do I have to apply for copies from the relevant birth/record county in the UK for the UK docs?

Has anyone managed to get a copy from the Zimbabwean government of their birth certificate? If yes how?

Any help here would be very gratefully received.

Thank you
Cadburys

Zimbabwean duplicate full birth certificates.
Order certified copies of your parents’ birth and marriage certificates.

Also include evidence of your three years’ residence in the UK prior to children’s birth.