But you shouldn’t be allowed to go home. See here for an article on how fighters for ISIS in Syria who came from Britain are now disillusioned and want to go back home, but are afraid of being jailed. They are disillusioned because they claim they are fighting other rebel groups and not Assad’s regime, but there is no good reason for letting them come back.
After all, why should we believe them? ISIS is a vicious organization, and anyone who went to fight for them presumably knew how vicious they were, yet chose to join them anyway instead of some other rebel group that wasn't vicious. So we have to assume they are lying and want to come back in order to engage in terrorism in Britain.