I dont think the below paragraph out of the ERM allows for much interpretation...
2.1
The Company maintains a zero tolerance approach to the consumption of alcohol and drugs, and therefore employees should not be under the influence of alcohol or drugs while :
carrying out duties on behalf of the Company - on or off Company premises, during official breaks (including meal breaks), and on periods of standby subject to being called on duty
in uniform (on or off duty)
driving a Company vehicle (on or off duty)