I've done a quick search to make sure I'm not double-posting, couldn't find anything about GTA IV being this cheap.
Xbox 360 URL - http://www.argos.co.uk/static/Product/partNumber/8950279/Trail/searchtext%3EGTA+IV.htm
PS3 URL - http://www.argos.co.uk/static/Product/partNumber/8950097/Trail/searchtext%3EGTA+IV.htm
Description from Xbox 360 version:
Pre-owned: Grand Theft Auto: IV Xbox 360 - 18+.
Autumn/Winter catalogue price when bought new £17.99
Suitable for Xbox 360 console.
This is a pre-owned game.
All pre-owned games are in full working order and of satisfactory quality.
What does the American dream mean today?
For Niko Bellic, fresh off the boat from Europe, it is the hope he can escape his past.
For his cousin, Roman, it is the vision that together they can find fortune in Liberty City, gateway to the land of opportunity.
As they slip into debt and get dragged into a criminal underworld by a series of shysters, theives and sociopaths, they discover that the reality is very different from the dream in a city that worships money and status, and is heaven for those who have them and a living nightmare for those who don't.
For ages 18 years and over.
This item is FREE delivery within 48 hours.
I have also found CEX to have the 360 version available at £10 too, and the PS3 version being on-sale at £8. Personally, I would prefer to buy from Argos as I would only trade things in at CEX. But if you're interested, here's the links to those as well:
GTA IV 360 - http://uk.webuy.com/product.php?sku=5026555245920
GTA IV PS3 - http://uk.webuy.com/product.php?sku=5026555400213