What are my chances of getting the police to allow me to bolt them securely against the wall with suitable locks and security cable?
It’s not an absolute requirement that guns be kept in a cabinet but for most people it is the practical solution.
One of the commonly accepted alternatives is that they be kept in a proper gunroom.
I suggest you talk to your local police and discuss the possibility that your office be turned into such a room.
In principle I see no reason why it should not be.
The downside is that only you or other certificate holders would be able to hold the key to the room.