Microsoft launched a Windows Upgrade Offer program which allows users that have purchased a Windows 7 PC (that includes tablets, desktops and laptops) until the 31 January 2013 to get Windows 8 Pro for only £15 with 90 days of Microsoft Technical support for free.
The computers must be new (no refurbished models apparently) and can come with any flavours of Windows 7 (that includes Windows 7 Home Basic, Premium, Professional and Ultimate and must be registered before 28 February 2013.
Oddly enough though, the program's FAQ says that each upgrade license may be installed on any PC with a valid base license to a qualifying operating system (Windows XP SP3, Windows Vista or Windows 7).
Which could mean that (a) a full version of Windows 8 Pro will be offered (b) the "upgrade" may be installed other computers. Up to five upgrade offers per customer are allowed.
An email with a promotion code and purchase instructions will be sent when Windows 8 Pro becomes available and users will have to download and install Windows 8 Pro by themselves. An optional installation DVD is available for a fee (which has yet to be specified).
Source : Microsoft