​Whilst a firearm is in your possession and not being used, it must be made temporarily inoperable. This means it must be stored in such a way that a projectile cannot be fired. 

This generally entails making the firing mechanism inoperable e.g. removing the bolt. When the firearm is required for use, the action may be restored which renders the firearm operable.

A firearm is made temporarily inoperable when:

(a)  for a firearm designed to allow its bolt, breech block, firing pin or other
      integral part of the firing mechanism to be removed – if the part is 
      removed and securely stored separately from the firearm; or

(b)  for another firearm – if the firearm’s trigger is secured by a trigger lock
      and the lock’s key is securely stored separately from the firearm.

Section 8 of the Weapons Act 1990 provides information as to how a firearm is made temporarily inoperable.