Insulation of extension toilet

Hi guys,

I do apologise up front for another topic regarding wall insulation but I'm completely lost due to different approaches on the matter. I will try briefly explain what I want to do.
I have an extension at the back of the house with small toilet at the end. Internal walls are painted bare bricks with flat felt roof (no ceiling). I want to convert it into proper toilet, insulated, with tiled walls and floor, plus heating.
My concern is damp, as I want to have the toilet insulated from the inside. External insulation cannot be done, unfortunately. My question is, how to make insulation that will work properly?
All together I can spare max 100 mm of space for air gap, insulation, plasterboard and tiles, because space for the toilet is very limited. I drew basic layout of the house and extension and marked the external wall which is solid, brick wide wall.
Is it safe to place battens to create 20 mm air gap and attach 50 mm PIR board with plasterboard? Do I need VCL membrane between insulation and plasterboard? If yes, do I have to this on every other wall as well insluding ceiling (I was thinking of spot lights)? I do not know if the entire room must remain air thight?There will be an extractor fan on external wall. I'm not sure if this will cause any issues for insulation of the wall around.
There will be no bath or shower, just toilet and small sink.
I'm about to start calling contractors about quotes but I don't want to be simply ripped off and left with damp in the toilet if they do it the wrong way. I'd rather ask over here to have some basic idea before making any final decisions.

I do appreciate any responses on the matter.

Many thanks