Fortunately most posters here agree that the UK should take her back and deal with her. Its worth mentioning that this is extremely difficult and courts are not properly equipped to handle cases like that. Still, there are no good alternatives. One of the most important goals should be to help the victims of ISIS to get some semblance of justice. When I am talking about victims I am not include Shamima Begum, but the people in Syria and Iraq who were killed, abused, enslaved and robbed by ISIS regardless of their religion, ethnicity, gender or any other characteristic. Foreigners joining ISIS are particularly galling, because they exacerbate a horrible situation for the population of these countries. Our opinions about the mental capacities of 15 year olds and her actions differ too much to find common ground.
Formal justice for ISIS perpetrators is going to be difficult, because individual accountability is more or less impossible. The British prosecution is hardly going to find witnesses in Syria, who could testify. One consequences of this is that her victims are even less seen and have no voice at at all to tell their story. They won't have their day in court even if the justice system prosecutes her. Considering how flawed the situation is, painting voluntary ISIS members as victims is imo an insult to the people who suffered under ISIS.