Question about citizenship application

Good day everyone,

Apologies if this is in the incorrect subreddit, I couldn’t find anything else relating to the topic. If I am please kindly guide me to the correct sub.

I will be going on honeymoon with my fiancé next year within Europe. I am a German citizen so this isn’t an issue for me, but we both live in South Africa, and she is a South African citizen.

My fiancé’s mother was born in the UK. She has a UK birth certificate, but didn’t apply for the citizenship. I read that this automatically makes my fiancé a UK citizen, as she is first descendant.

We would like to apply for her UK passport, but we are concerned because her mother never registered, will she be required to register first or do you think UK will accept the birth certificate for my fiancé as well as her own as proof of daughter, and register her?

I would just go through the application but in South Africa the amount of money we have to put forward for it to just get rejected is pretty steep, so we would like to be prepared.

Any guidance is appreciated.

Thanks!