flooring an insulated loft space?

As the other answers say, it's still "Storage only" unless the floor is rebuilt to proper structural standards.

The way I've done my last two houses is use "2x4" CLS on edge, crossways on to the existing ceiling timbers. (CLS = studded wall / drywall timber. The actual side is more like 1 3/4 x 3 1/2 inches).

I double them up side to side where the edges of floor panels will be and use staggered/overlapped pieces all the way through; at least 3' overlap to guarantee no extra flexing at the joints.

I drill a 12mm hole half way through at each intersection with an original timber, pilot through and use a 4" screw to pull the two tightly together, plus hanger straps at the ends where they butt up to a wall.

You can re-fit a layer of insulation between the new timbers before putting the boards down.

With everything screwed solidly in place the floor is as rigid as a conventional one and I've no traces of cracks in the ceiling below after 15 years with the present one, despite regular use and lots of storage.

Ideally also add insulation (between the timbers) and lining to the underside of the roof; that adds to the overall insulation and prevents too extreme temperature changes in the loft.

im going to assume its a uk question ....if so ..the ceiling joist are not big enough to make a floor ...most are around 4" ..100mm tall ....needs to be 8" 200mm ...you cant just add to top ...a loft is for storage only ...uk insulation needs to be 11" 275mm minimum ...if you want to floor for storage you can increase thickness of joist ..screw only ..no nails ..you can change insulation to 100mm celotex ...but do not compress the insulation ..it can make ceilings bulge