Downlight wiring in loft, Insulating loft

Hi

I have ordered the celotex board insulation to put in between the rafters and floor, the rafters are exactly 120 to 130mm and same is on the loft floor.
I am planning to cover it all up with 100mm board and then board it up.
My first question is would 100mm board insulation be enough. I am putting another 100 on the roof rafters.
Secondly, the timber on the floor is about 40cms apart, if i lay board across it, would it be strong enough to take the load, I Am only planning to use it for storage.
The third and most important thing is that initially I thought I will leave some space around the downlights and cover with downlight cover but my problem is that the wiring for all this is going over the timber and not through a hole in the timber.
The downlights are all fireproof but what should I do with the wires.
I dont have pictures but I hope I have explained myself, they are just going across the timber on the top.

Question 3) Generally cable for lighting is no less than 1.00mm CSA and rated at 16A without applying any de-rating factors. If the cable is enclosed in thermal insulation a de-rating factor must be applied. Worst case this would be no more than 50% therefore if the lighting circuit is protected by a fuse/MCB/RCBO less than 8A ( 3, 5 or 6 amp), it would be okay to enclose. If the breaker is greater than 8A (10 or 16A+) then the cable must not be enclosed or the cable size would require increasing.

So your options are dictated by the cable sizing and breaker rating.
If you desire and de-rating allows? Then you could hole or notch the timbers but the cable must be located greater than 50mm from top/bottom of timber, when holing, to prevent any potential nail or screw damage or safe plates fitted if notching.