Spray Foam Insulation vs Drylining (PIR Plasterboard)

I'd posted before about trying to insulate our 80s dormer bungalow. In a perfect world I would do both but right now budget allows for one only (foam or drylining). I've got quotes for 3000 to 3500 to spray foam the area behind the knee wall and we can dryline almost the entire house (external facing walls downstairs and all walls in the dormer area) for 2500. Of course each supplier is telling me I should do their solution first but I was wondering if anyone has any advice or if someone has done similar before?

My main concern about doing the spray foam first, is that this will still leave the pitched ceiling in the dormer area without insulation as there is nothing behind the plasterboard right now.

I've attached a rough image of what we have at the moment.