You normally need to maintain a 2" air gap above the insulation board, to allow for adequate ventilation into the roof space above your room.
To comply with current regs., you would need to have 3" Kingspan (or equiv.) between the rafters, and a further 2" underneath, running across the rafters; then the plasterboard over this.
Your spec. would therefore not strictly comply with this.
If your rafters are 3" deep, you will need to fix 2"x2" battens underneath them to give you the right depth for the air gap.
All this means that you will loose a lot of space, which defeats the object.
If one did work like this to a lesser standard than Building Regs, would it add greatly to global warming and climate change?
Whatever you do, for your own peace of mind, leave sufficient space above the Celotex or whatever, to maintain ventilation.