If only there was a forum about building regs where one could ask about it. Because it's quite complicated.
Do you know of one?
This one?
Building Regulations are not prescriptive, like those other easy regulations you mention.
What building regulations do is give a performance guide (the building shall be dry, the building shall be insulated, the building shall not fall down etc) and it’s up to you to prove that the your building conforms.
That’s why you don’t concentrate on one thing and don’t ask, “what insulation do I need in my roof and floor”, because it depends on the design of the roof and the design of the floor and how those interact with the rest of the building.
Taking Grenfell as a good classic example, you don’t concentrate on meeting regulation for insulation, and then have the whole place burn down. So you design a building to not only meet the required regulations individually, but also so that all the components work together. So your choice of 200mm of mineral wool, or 150mm or polystyrene, or 100mm of PIR as insulation will each meet the requirement for insulation, but will that work with the timber sizes, the, the cladding, the vented structure and prevention of fire spread?
You are building a commercial building for use by children and you appear to be bent on cobbling something together that you don’t have the knowledge of. So it seems that you need to employ someone who does.
And don’t rely on building regulations or the inspector to guarantee the work is correct and inspect it correctly. Inspectors have no liability that you building will actually perform as you say it will.