On principle i see no problems with it...its all about crims not having easy access to firearms, so in the roof is harder to get.
But...Depends on how you attach it...if attached to structural framework of the house through a wall lining, the only real access to the fixings to rmove the safe, is to rip the wall out...however if attaching to bare timbers in the roof, it may be easy to cyt a couple of them out to remove the safe...
At the end of the day, its about common sense, so dont worry yourself as to whether the particular timbers you are attaching to are load bearing as womble has suggested--that makes absolutely no difference at all--id hazzard a guess most safes installed in timber framed houses are attached to noggins & they arent load bearing--theyre just there to pick up the plasterboard...
Its all about how secure it is & that it is attached to the building structure...you dont want to be able to pull the safe out easy.
Aside from that, im with Tassie--going in n out of the roof space would drive me nuts...
Ring your local firearms branch, but id guess its all up to interpretation....so be smart about it, use common sense, as long as its firmly attached & not easily removed, you should be fine.
If there is easy access to the roof from the outside, id not put it in there because smarter theives of the older days would often gain access to houses vis the roof space as its pretty easy to lift a few roof tiles.