It should definitely be illegal, if it already isn't. Only last year the London Evening Standard reported that Tesco had been fined for selling out if date food at one of its branches. Mouldy apple turnovers etc. Private Eye had published a picture of a branch of Tesco with the newspaper on sale and on the front page was the headline that the supermarket had been fined for selling the inedible food.
However, I remember hearing on a radio station bulletin back in 1994 a story that a shopkeeper had been fined for selling a sage and onion stuffing mix to a customer. OK, nothing wrong there, except that the product was dated from 1969, presumably long before Best Before dates had been sued. It was so old that Paxo even wanted it for their archive. It only rang alarm bells when the customer noticed the pre decimal price on the packet and the very dated packaging.