If you hang these people they become martyrs so advancing 'the cause'.

The most appropriate sentence would be one of life imprisonment in general population with the ability for the parole board to release them on the understanding that they would be deported immediatley on release to Syria as that is the current country of choice for IS.

Thier UK passports should be revoked and they should be stripped of British residency.

That will make them stateless so they would need to be sent to Syria by the RAF and 'dropped off' at some quiet location in Syria to fend for themselves.

That will send out the correct message to others thinking of joining them.

That is assuming, of course, that they survive a few years in general population first.