Wire over or under loft insulation???????

My builder states he will be using 8 inches of "rockwool" type insulation - four running with the beams four running over.
I had thought it was good practice to
a: Cleat the cable to the ceiling joists for support, and b. Ensure the cable runs over the insulation to prevent overheating of the cable (related to fault current calculations).
In my situation above the two (a and b) seem to be contradictory, i.e. if I lay the cable over all the insulation then I have nothing left to fix it to and if I lay it under I run the risk of overheating and possibly having to upspec the cable to meet the calculations in the regs.
What do you guys do in this situation???????
Many thanks for all your help
P.S You have probably guessed what my bank holiday job is!!!!!!

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