She's a British citizen, so unless the government strips her of her citizenship.
Which it can do, but only for those who also hold another nationality - last time that it was done rendering someone stateless was during WW2 for a handful of cases of Brits joining enemy combatants. There's been a UN treaty since then strengthening provisions against leaving someone stateless.
Da'esh aren't going to deport her, and she cannot leave. Only if she escapes will she turn up here again.
At which point she can expect to be arrested, as membership of certain organisations (defined and redefined by the Government periodically) is an offence under the terrorism act.
Anyone who has lived with Da'esh can expect that on return, regardless of age, because of what they could learn in terrorist training camps. If you have the stomach for it, you can google for information. You don't have to go to the very radical sites (which is problematic in itself) to find sickening detail, including how children are trained to kill.