Most copies of Windows (90% or more) are purchased with new computers from OEM computer builders. Those builders use OEM versions of Windows which are not supported by Microsoft. The PC builder is responsible for support, including disk replacement.
When buying a new PC, I recommend making sure the PC comes with reinstallation media. If it doesn't, I recommend immediately obtaining such media or else building a system backup disk that you can use to restore your PC to its original state. Many PCs come with the ability to create a set of backup disks on your own. Since PC owners would be well-served to have a way to backup and restore their PC in case of malware or disk failure, this is something they should be doing anyway.
HP and Compaq PCs: Looking for Recovery Disks?
http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&cc=us&docname=bph07143
Waiting until you actually NEED backup or reinstallation media is a bad idea. Especially since, once the PC's warranty has expired, there seems to be no requirement that the builder even offer you restoration media.
Other than the Microsoft Store, the various Student programs, and the various Microsoft Subscription services (MSDN, Action Pack, TechNet), the only Microsoft-authorized source of downloadable Windows 7 is the Enterprise Trial version hosted at Downloads.Microsoft.com. That version is useless for permanent use unless you have Enterprise Win7 licenses.
Here's the MS UK page describing safely buying digital downloads of Microsoft software.
http://www.microsoft.com/uk/licensing/homeuser/buying-safely-online.mspx
"At the moment there are only two authorised sites in the UK where you can buy digital downloads of Microsoft software. One is Microsoft Store where you can buy Microsoft Windows®, Office® or Expression® software directly from us. The other is the Ultimate Steal site - the software there is only available to students and staff at educational institutions. If you want to buy any Microsoft software as a digital download you should avoid auction sites and peer to peer (P2P) file sharing sites."