Windows 8 - Great Deal £24.99 or £14.99 + Quidco
I don't know if it's already been posted on here but Windows 8 will be available for download for the sum of just £24.99:
Microsoft Store United Kingdom Online Store
I believe that this is the upgrade version and requires an existing Windows installation (7/Vista/XP).
Don't forget to link from Quidco, currently at 15% - so it could be a great deal!
Also, note that if you bought/are buying a Windows 7 PC (not starter edition) between 2 June 2012 and 31 January 2013 you can potentially get the upgrade for £14.99:
Windows Upgrade Offer
If you get the £14.99 upgrade you can apply it to ANY PC with XP,Vista or W7 apparently - check it out here:
Windows Upgrade Offer - Program Information