From the information I could find about the visa application process for the UK, they only refer to you needing a passport:
Regarding the German side, travel is only possible if you have been issued with a Fiktionsbescheinigung according to paragraph 81, section 4 of the Residence Act. This is issued in cases where a person already had a permit in Germany and has applied for it to be extended or for a different type of permit. You should check your Fiktionsbescheinigung and see if there is a reference to ‘§81, Abs. 4 Aufenthaltsgesetz’.
In other cases, it is not possible to travel with a Fiktionsbescheinigung.