Sadly, this isn't true. Online retailers can have their own T&Cs which form a contract.
Tesco, for example, can cancel an order at any time until dispatch - even AFTER they've taken the money.
No-one has successfully challenged this, afaik.
That may well be true, but as I can see it neither the retailer or customer has a tangible loss, therefore no need to challenge anything.
But again I would suspect that Tesco's cannot supersede the Law, no matter what they put in their 'terms & conditions'.