As far as I am aware it is not normally too much of a concern as long as the bathroom is on the top floor of the house (and the air is exhausted to the outside). It can be a big problem if the bathroom is underneath another habitable room as the celling would form part of the fire protection of the room above in that case. Or if the air is being extracted somewhere it could get back into a habitable space (as it would carry toxic smoke into that area), but as this is damp air you probably are just going to extract it straight to the outside. However it does depend somewhat on the design/construction of the building and local building codes.
It is possible to buy devices to close off the extract point if a fire occurs. These include fire cuffs and wraps that will crush the duct work in the event of a fire, and grilles and vents with built in fire dampers (that will close the vent if it gets hot). It is essential to use these devices if your ventilation system penetrates a fire rated part of the building, in order to maintain the safety of the building. If you are in any doubt, I would recommend fitting safety devices suitably rated to maintain the same fire protection that you had before you started (e.g if you make a hole in some plaster board, fit a damper/cuff that is rated at least as good as the plasterboard). Make sure you follow the installation instructions fully.
