I don’t know if it was the same but there was a Brit mini-series about 20 years ago that had the same scenario of the Germans having won WW2 and it was now the 1960s - Hitler was an old man who kept out of the limelight, the regime had hidden the Holocaust by destroying the camps and records. The Germans installed the Duke of Windsor on the British throne ad the US President was Joseph P Kennedy, a Nazi sympathizer. A cop in Berlin starts investigating claims of past genocide by the regime, which is in trouble to begin with due to domestic unrest. He finds evidence of their dirty deeds which leads to his attempts to expose the mass murders.
Sounds a lot like “Fatherland”.
There was also a movie made in the 60s called “It Happened Here” about a Nazi occupation of Britain.
Joseph P. Kennedy is scheduled to meet the fuhrer on his birthday, and he's handed a dossier with evidence of the camps.
He reacts with horror, and the Presidential limo drives away, leaving Hitler standing there looking foolish.
I didn't know it was a mini-series. And maybe I'm confusing it with another film.