Shooting legal advice
Reading Conditions 4 (a) and (b) of your shotgun certificate makes a good starting point.

Condition 4 (a) requires your shotgun to be stored securely except in the circumstances listed in 4 (b).

These include occasions when you are staying away from home and have your shotgun with you for the purpose of shooting with it, or when you are taking it to or from a place such as a shoot or shooting ground.

Condition 4 (b) requires that you then take reasonable precautions for the safe custody of your gun. This indicates a lesser but still significant level of security, and you would be well advised to read the Consolidated Guidance on Firearms Security published by the Home Office and the Association of Chief Police Officers.

Recommended security steps include concealing your shotgun, for example in the locked boot of your vehicle, fitting an alarm, removing an essential component, fitting a clamp or cable and, while staying away, selecting accommodation that provides security facilities.

As a responsible certificate holder you must ensure that your shotgun is secure in the particular circumstances.

As a matter of common sense, this is unlikely to include leaving it in a parked vehicle for days or weeks away from home.