Dear @Moro11,
Welcome back to the Wefugees Community and thank you for reaching out to us again.
From my point of view, your case is not depending on the German authorities and they won't deter you from applying for a visa for another country. Only be aware that your "Duldung" will be unvalid as soon as you leave the country. Instead, you will need to meet the requirements of UK's family unification procedures. Unfortunately, I am not an expert on UK's immigration rules and regulations - especially since they left the EU. I would recommend your partner to contact migration counselling organisations in the UK for individual support. Let us know if you need help in finding a respective office - I would be happy to make some research.
Regarding your second question: I would definitely inform the immigration office (Ausländerbehörde) if you have managed to get a visa for the UK in order to prevent potential complications in case you want to visit Germany in the future.
I hope this helps and please also don't hesitate to get back to us with any further questions.
All the best,
Meike