Concrete flat roof insulation from inside - ideas?

[3MagicBeers]

I'm building a house with concrete flat roof (20cm thickness, beamed & rebarred) with a total surface area of 190m2 (good waterproofing is taken care of).

 

Inside will be a plasterboard ceiling with plenty of LED down lights, fitted 20cm from the bottom of the concrete roof. 

 

I would like to cold roof insulate using SCG 7.5cm StayCool glass wool rolls. Warm roof insulation is unfortunately out of the question. I would like some advice on the best way to do so - using wooden joisting, do I fit the insulation snug to the bottom of the concrete roof? or fit the insulation with a 10cm gap from the bottom of the concrete roof? I am hesitant to lay the insulation directly over the plasterboard & the LED light fittings due to fire risk.

 

Any thoughts and advice is much appreciated, cheers !