The specs in the blurp state it's an Intel Core i5 2400S, which is the Sandy bridge processor included in the latest hardware revision. The 2011 iMacs came out in May, before Lion was released (July) so there is a chance it has SL installed. it will be the same hardware as the current iMacs, but may include the old keyboard that has a dashboard icon on F4. The newer keyboards have the Launchpad icon instead.
If it is you can upgrade the OS through the App Store and, other than the keyboard, it will be a latest revision.
TBH you'd probably get a better deal going through the [Apple] refurb store on line than Tesco.
I've also noticed a couple of online ads from PC World advertising 10% off the macbook pros and iMacs. this could indicate an attempt to reduce stocks for an imminent release of updated products.. but who knows!