mis-prices

It's a common misconception that misprices have to be honoured.

A price on an item in a shop (or website) is only an "invitation to purchase". No contract is made until both parties have agreed a price and money and goods have exchanged hands.

Shops can legally refuse to sell an item at any given price.

They're not misproced

The advertised price is a "invitation to treat". Until you make an "offer" to buy at that price and they accept your offer there is no contract. Play are quite specific about this, they have not accepted your offer until they dispatch the item.

Nor does this come under "bait and switch" which is where a product is adverised at a very low price but once a customer enquires they are told it is not available and are offered a more expensive alternative. The idea is that some customers will go for this as they've already spent time and money coming in and it's easier than going elsewhere.

@Tom, that's just Sainsbury's (and a lot of large company) policy. Theory being that in the scheme of things it's not worth the aggro to annoy a couple of customers. No retailer is obliged to sell anything to anyone at any price. If I don't like your face I can refuse to sell you anything. Until I take your money it's mine and I can do what I like with it. My shop is also private property and I can ask you to leave for no reason.

What I'm trying to say (in a roundabout way) is: If you don't like the price on offer you are at liberty to buy elsewhere.