Installing Pvc panels behind the tub but the wall/tub isn't level

I'm planning on installing some pvc panels but the the tub or wall aren't level. The tap side is okay but the foot side has a pretty big gap. In not sure how far out the boards will come after they've been fitted so unsure if I need to bridge the gap some other way.

The tub is pretty old very heavy metal one and all the plumbing for it is iron connected to and iron waste pipe so I really don't fancy moving it about. How should I combat this. Should I set up some studs for the panels or plaster the side where the gap is too much? Or will the panels plus adhesive cover that 2.5cm gap?

Many thanks