Warm roof- plasterboard or not?

[Crofter]

I've got a warm roof construction with a pitched roof. In part of the build this forms a vaulted ceiling, and in the rest it will become attic space. My gut feeling is that I should just plasterboard the whole thing, to leave a neat final surface that hides and protects the VCB. There is also the fire protection issue, where plasterboard will protect the frame (well, rafters in this case) from damage for a period of time,

 

Just curious about what the normal practise is? I have a feeling that boarding vaulted ceilings will prove to be a right PITA of a job so just want to check that it is really necessary or at least a good idea to do this even on areas that will not be seen.