It is generally a gross misconduct offence to be drunk whilst at work and in a bar its certainly a breach of H&S to be handling glass and/or machinery of any kind whilst intoxicated however most bars are flexible in allowing staff a half behind the bar or during their break...depends on the bar and brewery rules
Since you've posted this question under 'Law', the simple answer to your question has to be 'No'. There is nothing in any of the Licensing Acts which prohibits bar staff from drinking alcohol while at work.
There may well be 'health and safety' considerations to be taken into account but these are only covered by the same rules as any other occupation.
The health and safety regulations would be that in the eyes of E.H.O beer is classified as food, so you have to act the same way you would in a kitchen really.
I have heard of a by law which bans drinking alcohol and smoking behind a bar - local H&S and Environmental Health might be interested in the establishment ?