It depends on either of two things:
The terms and conditions of the seller.
When selling online companies can state that a contract is not formed until the item is delivered. This is perfectly legal and means the seller can cancel the order at any point up until delivery regardless of whether they have accepted payment.
The price. If an item usually selling for £1000 is advertised for £100 then it should be obvious that that is a genuine error, unless there is something in the advert that makes it clear there is no mistake. In this case the company can cancel the order.
If the difference is minimal then unless the terms and conditions state otherwise they may be bound by it.
It's worth knowing that some very big companies such as Argos, Tesco Direct and Screwfix have all been able to refuse to supply goods that have been advertised at the wrong price even after they have accepted payment.