You need to stop the humid air getting into the loft if you don't want to do ventilation. You could do that by improving the ventilation in the bathroom (opening a window as well as running the fan. A big fan won't do much if the air tries to remove doesn't get replaced), generating less water vapour in the house (do you dry washing on radiators, boil lots of water in open pans without extraction etc), or you could try running the dehumidifier inside the house rather than in the loft so that the air that leaks into the loft isn't carry as much water.
