It might be sensible policy, though, not to sell alcohol to those whose uniforms imply that they might be driving - train, bus or taxi drivers perhaps - as licensees have a responsibility to prevent crime and protect public safety. Licensees have a DUTY to refuse alcohol those who are drunk or underage, but they have the RIGHT to refuse service to anyone unless it's on the grounds of protected characteristics under the 2010 Equality Act.
Looking at the reader comments on this news story, it's interesting that a few people have weighed in to say that the old law still applies. Since the 2003 Act was enacted in 2005, it's been mandatory for all new licensees to gain a licensing law qualification. Prior to that there was a grace period for existing licensees to transfer over to the new system under "grandfather rights" without being trained. Whilst there might still be a few independent licensees around who are blissfully unaware of current legislation, I'd like to think that large managed estates like Harvester are on top of staff training.
I always round off licensing law courses with an action planning session encouraging learners to think about how they're going to pass on what they've learned to the staff they supervise. Many offences under the 2003 Act can be committed by members of staff as well as license holders, and responsible supervisors should give staff all the tools, training and knowledge they need to work safely within the law (and avoid causing such negative publicity). If this story is true, then the manager was wrong, but perhaps his employers need to ask themselves why he based his decision on urban myth rather than fact.
(Thanks to @BrynBella for the heads-up about this story)