A bit of both. For a complete refurbishment the best installation order is:
Install the bath
Install the panelling
Install the basin & toilet
Remove the existing suite, install the bath in accordance with the manufacturers instructions to ensure it is totally rigid (there needs to be zero movement when you stand in the bath). Completely seal the bath to the wall(s) so that no water can get down the joint, using silicone sealant. Next fit the waterproof wall panelling, and apply a second bead of silicone along the joint between the panelling and the bath. Continue fitting the panelling around the rest of the walls. Then fit the wash hand basin and toilet.
If you are using built in bathroom furniture, fit this before the panelling.
If using wall hung basins or wall hung toilets special procedures have to be followed. Contact us for more details.