I had my gun safe and ammo box raw-bolted into an outside in the under stairs cupboard in London and the MET FLO’s were entirely happy with it as it was very secure and hidden away from prying eyes.
I’d avoid the loft/attic even with a dehumidifier as shotgun ammo doesn’t like large temp changes.
Yeah my thinking - I can coach bolt in both locations. Only thing is my wine stock would have to move