RJH wrote:
Yup. I did this when I had the attic rooms ceiling down. I did the
calculations at the time and worked out 50mm was the most efficient in terms
of cost, insulation and ventilation. And for a storage space/one you're going
to be walking in, so much easier than stilts/boards etc.

It's not impossible to have a mixed plan - use PIR boards for the parts
don't want to raise, and fibreglass/mineral wool for the parts you don't
care about.

Most PIR has a resistance double that of mineral wool, so 50mm PIR is
equivalent to 100mm mineral wool. If the OP wants to go for 300mm of wool,
that means 150mm PIR. I would probably compromise a bit to make the heights
work - maybe 90mm PIR between 100mm joists?

Theo

(I'm currently replacing patchy wool between 120mm deep rafters with 50mm
celotex, for coverage, breathability and improved heat reflection. Further
down the road I'll look at 100mm celotex under the boards between 120mm
joists)