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Thanks for that ZM, although having found the relevant legislation I'm still somewhat confused, because it refers specifically to sale of alcohol, and "does not apply in relation to any sale of alcohol which takes places on the premises merely by virtue of being treated ... as taking place on the premises," which by my interpretation seems to imply that if I am not the customer then my presence should have no effect. It still seems like there is an element of over-zealous in-store policy going on, which could in principle be challenged.
On the other hand, the website points out that there is "no automatic right" to enter the premises anyway, so maybe I'll just have to accept this after all. Still, it's rather frustrating.