If they are on a stud wall, just smash through them & get it back to the stud then re-sheet. I take it its not a large area being only one bathroom wall? You will be exceeding weight limits of your plasterboard tiling on top so best to re-sheet.
If they are on a solid wall & you are happy that the wall is square enough to tile from & tiles are sound, then & only then tile on tile. Double check for blown plaster. You will hear this by tapping on the tiles. Clean up your tiles with sugar soap, scratch the surface of the tile with an angle grinder to give you a good key then coat with an SBR. With the tiles you have chosen, you will be able to tile on top using Bal Grip (some prefer Bal White Star or using an cement based SPF. Depends on your tiling speed)
For your other 3 walls (and your forth if you are replacing the stud) sheet out with 12.5mm plasterboard. Your PB can take up to 32kg pm2. I know you are a plasterer & your natural instinct maybe to give it a skim but don't. You will be reducing your weight limit down to 20kg pm2. Plus you only have 2 weeks of so drying & tiling times are a no no. A 12mm ceramic tile inc adhesive & grout will weigh in at 28.52kg pm2 just to give you an idea.
