My force seems to agree with Codling's. Better off having a cabinet where it'll get used.
Don't forget your floor will (probably) also have joists, and these will do just as well as those in the loft. All the police would like to see is that it is securely attached to the fabric of the dwelling, and out of sight of casual visitors. As ever, common sense applies.
If making the cabinet blend in with the surrounding is the concern, one tip might be affix it on it's back against a wall, perhaps hidden under a piece of furniture such as a bed, then if you want glue carpet, laminate or whatever type of flooring you have onto the cabinet so it blends in. To the casual observer this will look as though it's a covering for messy central heating pipes or whatever.
Also, laying the cabinet on it's back means that you'll be able to fit more in.