For years and years this was the standard way to finish-off a bathroom ceiling - plaster the surface (either over laths or plasterboard) and then a coat of emulsion paint over this.
The big problem for homeowners is that neither of these products is ideal for use in the moist conditions of bathrooms. These days there are specialist paint coatings that have a much better tolerance of the conditions. There are also newer coverings such as ceiling panels which offer a surface that is impervious to condensation, moisture or even direct water spray.
YOU NEED A SPECIALIST BATHROOM PAINT SUCH AS THESE - ORDINARY EMULSION WILL FAIL