danny - // I could understand this if he had fought against British forces, but he didn't, he fought against ISIS. //
The point is, he fought for an organisation that is classed as a terrorist organisation under British law.
Anyone can find a justification why any law, large or small, should not be applied, and there seem to be a number of them posting on here.
Just because there is a perceived justification for this man's breaking the law, it does not excuse the fact that he did break the law, and there are consequences for that.
No amount of huffing and puffing about 'gentlemen being against decapitation' and similarly emotive, but irreverent nonsense detracts from the fact that we enforce our laws, we don't just enforce them when we decide that one person should be given a pass because we may like his motives.
It could be argued - and often is - that the law is wrong, but that does not prevent its application where it is in force, and this in this situation.