Update: I checked the website and saw that it actually comes with Mavericks (10.9) from a reviewer. The person may be wrong, but many people have up-voted this post.
OS Mavericks, excellent delivery, outstanding price, no hassles.
By John - 20 Nov 2014
Amazon Verified Purchase
Delivery was the following day. Very straightforward buying experience. Excellent communication on delivery throughout.
The product came preloaded with OS Mavericks. I easily upgraded to Yosemite.
It is an UK model with UK plug, wireless keyboard and magic mouse in a sealed box.
I feel there is no point in commenting on the actual item, as that has been done better elsewhere. (I would never want to use Windows again having said that.)
The same product was £899 at Currys / PC World. I saved £160.