There have been several cases where successful prosecutions have been brought against drivers for eating a Mars bar, or drinking a can of Coke, while driving. So seeing a driver taking a drink from a can (even if was a soft drink) would entitle the police to stop the driver, and to breathalyse him, due to him having committed a moving traffic offence.
Although people often talk about the offence of 'drinking and driving', the actual wording of the law refers to 'driving while under the influence of alcohol or drugs'. Clearly, in the case you describe, the driver would not be 'under the influence' (as recorded by the breathalyser) and no arrest or prosecution could follow. (Except, of course, that the driver would probably still be prosecuted for 'driving without due care and attention' or whatever other charge the police, together with the Crown Prosecution Service, deemed to be appropriate).
Chris