Hi
@ash_wath,
I could not unfortunately find any information about whether it is possible to obtain a visa for the UK with a German type D visa. You can contact the British authorities with questions about visas here:
https://***.gov.uk/contact-ukvi-inside-outside-uk/y/outside-the-uk/something-else/english
As for the Fiktionsbescheinigung, this would not change the situation. A Fiktionsbescheinigung in your case would mean your visa continues to be valid after expiry until the Ausländerbehörde have come to a decision on whether to issues you with a residence permit.
Depending on the visa processing times for the UK and the status of your application at the Ausländerbehörde, you could start the process and then hopefully have the permit in time to submit it to the British authorities.
Best,
Éanna