That is indeed the law in the UK, and applied to this story. The key thing here is that the customer ('MM') explicitly entered their side of the contract by clicking 'purchase' and providing payment details on HP's website. HP could have declined at this point: their original email and special offer website was only an 'invitation to treat'. But in sending an order confirmation message to MM, HP had chosen to sell - and thus made a valid contract.

Perhaps HP should have a human representative approving each offer and entering into contracts manually. However, I can't see that being likely or (from HP's point of view) entirely satisfactory.