You need to read the Railways & Transport Safety Act 2003, which now applies the law to the consumption of alcohol on duty and during stand-by, etc.
http://www.opsi.gov.uk/ACTS/acts2003..._20030020_en_1
Part 5 is the bit you need.
In a nutshell the limits are roughly 1/4 of those for driving so as long as the 'accused' does not exceed the limit no offence is committed under this Act. But sniff 1/2 a lager from 200 yards and you'll more than likely be over the limit.
Unless a person is convicted of an offence under this Act I cannot see how any disciplinary action could be taken by an employer, otherwise the flood gates could be opened to all sorts of abuse.
But I do agree with the sentiments regarding drinking whilst in uniform - it can easily cast doubt on one's integrity. All Joe Public sees is the uniform and will always draw the wrong conclusion.
Now then, one week off so where's me beer?