This is what I think (if I still remember from last night's viewing!):
The Human Rights barrister (Hannah) got involved in a plot with a bunch - that also includes the young soldier's male solicitor - to stage an incident that would prove that the previous terrorist case (the one where Rachel, the female policeman's lover is involved in) was staged and an innocent man was sent to jail. His sister is also involved with this group.
So they faked the CCTV that showed Sean assaulting Hannah. The plan was that Hannah would remain hidden and then it would be unveiled when Sean went to Court that CCTV could be faked (and that the terrorist one was faked), and she would reappear in a big reveal.
Then (I'm a bit fuzzy here
) - something to do with the CCTV video of the assault being leaked online - the CIA got involved with the British intelligence, and decided that Hannah had to be disposed of so they found out where Hannah was hidden, and killed her
so that their fakery in the terrorist case would not be exposed.
Rachel (the female cop) has figured this out and she now knows that Sean is innocent, and her and Sean have teamed up to expose the whole caboodle. Plus Rachel was the one who alerted the CIA/Brit intelligence unwittingly, because she was asking her old team in the intelligence department, for help and feeding them info as to how the case was progressing.
This is as much as I know, from faking full-on attention whilst also knitting.