It depends very much on the FLO. Mine prefers the cabinet to be on the same floor as you sleep, more secure at night, but that is only a preference, mine ended up in the loft (a higher authority objected to the cabinet in a bedroom).
They don't like 'outbuildings' which includes garages but if you can convince him of its security, as yours is part of the house he may accept it.
The weak point will be the car entry door, always the easiest to force. Concentrate your efforts on securing that, an alarm may help, and you may have no problems.
There are no rules as such, the legislation doesn't even require a gunsafe or cabinet, just that you take "reasonable steps to prevent access by non certificate holders". My uncle had a side plate hammer gun and used to take the locks off and keep them in his safe, the rest of the gun lived in the wardrobe. A gun safe is probably the easiest way though.