Legally its yours, so i’d keep it
1)They put the item up for sale at £x.
2)You said i’d like to buy that item for £x.
3)They said thats ok, and by posting the goods they accepted the transaction and passed ownership of goods over to yourselves.
They could have noticed the error at stage 3 and declined the sale (as per Slacks link), but decided to proceed.
Doesnt matter if its an internet transaction or shop, the laws the same in this instance. If it was a small retailer i’d think about returning it for the good karma. If its multinational i’d tell them to suck it up, point them to the case law and advise them to update there sales systems (or just ignore the email).