Installing plastic panels.

If the tiles are in a sound condition and bonded to the wall well I would fill the gap with something of a suitable thickness like 6mm ply bonded to the wall with plaster board adhesive and set level with the existing tiles, you may find that if you remove the tiles that they bring most of the plaster with them.

Adding the panels on top of the tiles could cause you a few problems, mainly due to the extra depth of material you are adding on top of the tile. Edge trims may look awkward Taps may become too close to the new wall Drainage from the wall into the bath and the look of the bath may be an issue if the bath has a narrow rim Overall, I would remove the tiles. Recommend you put a bit of ply on top of the bath while you are doing this to protect it from falling debris

to be a good job needs to remove the tiles. adding a plastic panel on top of the tiles it won't stick 100% and you can have problems in the future. in order I suggest you to rip out the tiles making good the wall and after all you can apply the plastic panel.

Hi if the tiles are sound you can leave these on, just degrease them. If you use ply to fill in the gap stick with silicone or gripfill. If the wall has a doorframe in then its best to take them off as it won't look too clever around the architrave.

It’s fine to do what he want as long as the tiles are clean ! Use 675 to sick the panels Regards Cosyhomes

Good enough answer but any packing would be better to be moisture proof although the finish surface will be moisture proof and it would be better to strip all and reboard or dot and dab sheets depending on weather the substrate is sound and not friable