UK law is clear in that ownership has not transferred to the recipients and that the items should be returned.
I think in this type of situation, it's a reasonable expectation that the recipient should return the PS Vita too. They paid £19.99 and got sent a completely different item; it isn't as if the PS Vita was priced at £19.99 in error and the company mistakenly fulfilled the order.
Sadly, I see similar situations happen all the time. Companies make a mistake with their pricing online and don't fulfil the order and the people who thought they were getting a 40" TV for £50 start talking about their "right" to buy it for that price.
It's an obvious mistake by the retailer and if their customers are being uncooperative then they have every right to pursue the legal avenue. Let's turn it around a bit: if the customers had asked to return the game they bought and accidentally sent a PS Vita to the company, would the customers be arguing that their mistake represented an "unsolicited gift"?