No, it wouldn't be illegal to try to benefit from the mistake, but the price on a label or a website is not an offer in contract. It is an invitation to people to offer to buy; what is called, quaintly, "an invitation to treat". And if a contract for sale was made on the basis of some fundamental mistake of fact, such as here, the contract is avoided by the mistake. M & S are entitled to retrieve the goods transferred under the agreement or to claim for their value, in such a case.
You may be wondering whether it is theft by the person who orders the goods, hoping, knowing or expecting that they will not be charged the price. The key word is hoping; it is not guaranteed that they will not be, and they would be taken to expect that M & S could claim the goods or the value It is not dishonestly appropriating M & S property with intent permanently to deprive M & S of it, at least not so clearly so that anyone would prosecute for it