Asda presently have an offer on half bottes of wine - any 4 for £5.
However, the tills do not seem to be set up to recognise this, so they charge the full price.
Just pay up and walk along to the customer services desk. They will refund the overcharge and give you a £2 gift card for your trouble.
This certainly works at the slough store, and I would expect that the glitch is nationwide.