If she has a valid passport she has every right to come here. If she has broken some law or if she breaks one when she gets back - then she will have that to deal with.The only information I have on this is the conversation with my daughter in law last night. The facts as I understand them are .
A 15 year old child leaves England and travels to a war zone. Her motivations were ??. Excitement? Love? , Enthrallment?. Confusion? Religious fervour? She spends 4 years there,, witnesses and maybe participated in vile practices. How much of this was voluntary,how much under duress, ? How much was survival in the situation she was now in?.
Now she wants out and home. . We had trolling on this site about Stockholm Syndrome,.. well is this not a classic case.?.
My general view is that every person is entitled to citizenship of one state,and one state only. I see the concept of a person with multiple passports and citizenship of two or more countries as bigamy. Her state and citizenship is UK.
My view also is that where an adult ,decides to leave their state and fight with and for another state, they should be free to leave, and their citizenship of their original state is forfeited. However removal of a citizenship is a judicial not an administrative matter. However at the time of her departure,she was a child,and incapable of making that decision.
That the UK or the US or even the EU would seek to punish people for vile deeds carried out in lands beyond their borders is an unbelievable arrogance. So is using terminology like terrorist ,
So my view is that she would be entitled to UK consular assistance,and presumably NHS medical assistance for repatriation. Same as would be offered had her aeroplane from Heathrow to Perth, crashed in Fiji and she spent 4 years with cannibals. ( Help Not trying to be racist about Fiji..just making a point )
Otherwise - we're a country of laws. That's how it works.