At the time she went to join ISIS she was a minor, so her age at the time is absolutely relevant. She was then abused by ISIS, she is a victim of ISIS. She's just come out of an incredibly abusive, traumatizing, and dangerous environment where she was brainwashed into extremism, so yeah it's going to take time for her to realize the weight of what's happened to her and regret what she's done and what has been done to her. Britain literally has deradicalization programs that don't require the commission of a crime to be put into, so she can be placed in a deradicalization program, given mental health treatment, and evaluated from there.
Britain has designed a system for this exact issue, and people are advocating they not use it in this situation because they want to sate their blood lust and shes an easy target.