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"The legally binding contract is complete when a retailer accepts an order. However, acceptance does not necessarily happen at the point of order. Even the confirmation email may not be an acceptance. Some retailers reserve the right to cancel an order up to the point of delivery. It is therefore important to carefully check the retailer’s terms and conditions (which must be available on their website) and emails – if a retailer simply acknowledges an order, there may be no contract at that point.Oh.
"After placing an order, you will receive an email from us acknowledging that we have received your order for the Goods, and the delivery services you have requested from the delivery service provider. Please note that this does not mean that your order has been accepted. Your order constitutes an offer to Evans Cycles to buy Goods. All orders are subject to acceptance by us, and we will confirm such acceptance to you by sending you an email that confirms that the Goods have been dispatched by the delivery service provider (the "Dispatch Confirmation"). The contract between you and Evans Cycles will only be formed when we send you the Dispatch Confirmation."
http://www.telegraph.co.uk/finance/personalfinance/money-saving-tips/10602641/Price-glitches-Do-retailers-have-to-honour-pricing-mistakes.html
"unless it has deliberately misled customers, it will usually be legally entitled to cancel the order right up to the point of delivery. Whether it chooses to do so is a question of public relations, not legal rights.”