Unfortunately, Parliament ruled out the option of hanging them for treason some years ago. Maybe the government could formally declare war on Isis, then our troops could legitimately shoot them on sight as combatants?They should be let back - then dealt with.
They're British citizens and therefore are our responsibility - we shouldn't be dumping them on other countries to deal with. Whether they're jailed, whether they're hanged for treason etc is a separate argument - we're British, they're British - we deal with them.