For a trip to CI the simplest thing to do is send the GAR to the local special branch ports unit (contact details on the GAR form ) and the NCU email address also on the form.
You need to do that at least 12 hours in advance and for both ways and if you change plans so that you will be leaving or turning up early, send them an update. If you are coming late don't bother, they can wait if they want to see you ;-)