Hi @Gago,
thank you for reaching out to us again.
I assume that you have received a Fiktionsbescheinigung according to §81 Abs. 4 AufenthG? In this case, you are allowed to travel with your valid travel document or passport to the UK, as long as your Visa is still valid. I hope my answer helps, otherwise feel free to contact me again.
All the best,
Julia