Roof insulation in new detached garage

[AlanUK]

Hi there, 

 

I have had a garage built, with an office room above.  I now have to insulate it, once the electrics are done.  The Building Control guy said I need to use 125mm Kingspan/Ecotherm etc PIR, and then nothing else would be needed.  My original builder said to put Kingspan in (but not as thick), and then use insulating panels under the plasterboard.  I prefer the BCO aproach, as the room above is small enough, without putting additional panels in to make it even smaller.  Is either option correct?

 

Also, when putting the PIR in, do I go right down to the wooden beams that are sitting on the concrete?  (see second picture below), or do I need to leave a ventilation gap anywhere?

 

Thanks.