Thanks for the compliments, I’ve looked at the drawings you have given and all looks normal. Just bear in mind that bathrooms are high humidity areas so vapour management is very important.
First a warm roof construction is the way to go and the vapour control should be under the insulation, I note on the drawing you have given me it shows the Vapour control membrane closer to the outside!
When you place the new flat roof insulation on the roof, the void between the joists now becomes warm. Now start looking at the fascia and where the void joins the walls and the tiled roof; make sure that you stop any flow of air from these areas and you also insulate them. If you don’t you will get cold bridging and possibly sweating.
These videos should help: