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IIRC this is the same thing as when something is priced incorrectly in a shop, eg displayed on a supermarket shelf at a lower price than it should be. Contrary to popular belief the retailer is not obliged to sell it to you at that price, they can simply withdraw it from sale. (Though, for the sake of customer relations, many shops would let you have it at that price and then go and correct their mistake.)
I’m showing my age a bit, nowadays they’ll just scan the item at the checkout and tell you that’s not the price.
You (and most of us) need to read about English law - it's the branch called TORT. Putting an item in a window with a price is just an "invitation to trade" from the seller, it's NOT a binding offer, which is a shame :-( Surprisingly, it's you that makes the "offer" when you go in and quote the price, they can refuse your offer.