well ISIS evidently suspects many British volunteers of being spies as, rather amusingly, they are denied the opportunity to die in combat and they have them working on menial duties, like digging and maintaining the latrines, or as porters, carrying supplies to the front line, like they're pack animals.
Hence all the 'disillusioned' jihadists who now want to return to blighty. Since their stated aim is still to do us harm, we say "no chance".