dry lining

Not really sure what you're asking here, but I'm guessing you mean you intend fixing kingspan to the walls, affixing expanded metal mesh to these and then drylining over the lot?

You would be better off studding the wall out first with 3X2, insulating between and then boarding over, much cheaper and you could use cavity batts instead of kingspan, or if you wish, use 25mm kingspan on the walls first, then screw through these to affix your studwork, so that there's no cold spots, then use batts.

If its an outside wall, I'd fix insulation board on instead. Or if yo do batten it, fill the cavity with insulation.

You can do it any way you want. If you really need that extra 1" you could strip all the plaster off. Do you?

NT

That sounds like a recipe for a perfect level to me. I'd been trying to visualise how to get everything flat and square and 'by eye' (like the pros do) doesn't sound very reliable in my hands. As a toilet, I shouldn't have any issues with electrics, which makes a nice change!