As a journalist in the film points out it was well known at the time. The MI5 man says but now we know the locations.
The journalist replies we always knew since we heard the screams. End of subject (in my view) but the film meanders on pretending these are all great issues of state.
In reality, the practice was so well known it even had its own vocabulary (like extraordinary rendition).
The thesis of the film was that the dirty politicians knew about it all along (and of course if they read the newspapers they did) whilst MI5 was trying to hold them to a higher moral standard.
Ironically this is the exact opposite of the truth since to quote Wikipedia "A 2018 report by the Intelligence and Security Committee of Parliament found the United Kingdom, specifically MI5 and MI6, to be complicit in many of the renditions done by the U.S., has helped fund them, supplying them with intelligence and knowingly allowing them to happen."
Of course, 2018 was long after the film was made but it does show up the fact that while MI5 can't catch spies or terrorists, it is terrifically good at massaging its own image.
