Most fan isolator switches for bathrooms and WC are positioned outside the room above the entrance doorway. As far as I can see you only need to provide isolation for the fan the rest I would suggest is wired directly into the lighting circuit you describe. I do not know how you intend to create the cubicle but presume it will be a studded structure with a wall and doorway at right angles to each other. If so you should be able to wire in the isolator switch during its construction.

Richard