MacBooks are expensive. iMacs are even more expensive. Because buying an Apple product brand new is a very expensive way to do things - especially when there are much cheaper ways to do things. Ways that including buying 'renewed' (basically second hand) versions that have been put together so they're basically new.
Renewed tech is always cheaper than buying brand new, but today Amazon has knocked 20% off renewed/refreshed/whatever word you want to use MacBooks and iMacs. Specifically you can grab a Macbook Air with an 11.6-inch screen, 4GB of RAM, 128GB HDD, and OSX Mavericks for £311.50 instead of the normal £399.50.
That's £87.51 off the normal price, which is even less than it would cost to buy a brand new Macbook Air right now. Or even when this one was brand new.
Amazon promises that it works and looks brand new, and it has a one year money back guarantee if it does cock up.