Loft insulation and flooring has me stumped!

[ungod]

Hi everyone, I'm looking to insulate and floor my loft to use for storage and I really don't know where to begin. I've been reading through lots of posts here in the hope that it would enlighten me but still no moment of clarity.

Situation as it is:
House is three years old and the joists are 100mm and insulated with blown shreds of fibre. This is uneven in places and I reckon it's not enough.
I think the roofing felt is breathable but I'll check with the builders.
Not sure about a moisture barrier under the existing insulation so I'll also check this with the builders.

Hoping to do:
I'd like to insulate as much as possible but without increasing the joists, as this would be more than I'd be willing to undertake, or my wife would let me do.
I'm unsure if I should leave the loft as a cold room, or insulate the roof to make it a warm room to avoid extremes of temperature. Would this be of much benefit to items being stored?

Where I'm stumped:
If I compress 200mm of insulation (thinking of using Space Blanket rolls) between the joists under the flooring will it still be as effective as 200mm of uncompressed?
If it's not as effective, is there any alternative without increasing the depth of the joists?
Is it of any benefit to make a warm room if it's only for storage?
If creating a warm room, do I still need to insulate between the joists, under the flooring, or is insulating the rafters enough?
I'd prefer not to use tongue-and-groove flooring in case I want to lift any panels/boards in the future. Is this OK and what are the best options for the flooring, e.g. chipboard, plywood, etc.

Any advice greatly appreciated.

Thanks in advance,
Mel