Heads Up:
Anyone looking to install a fresh copy of Windows 7SP1, it no longer works 'out of the box' you need to manually download and install KB3102810 (An Update to Windows Update), otherwise it will endlessly check for updates.

There are two newer versions of this update, (Feb2016) KB3135445, and (March2016) KB3138612, but these seem to break image restores of Windows 7SP1, i.e. restores from Paragon HDM 2015 or Acronis. The restored image of Windows 7SP1 works, but Windows Update won't - it will endlessly search for updates/fail to find updates quickly. KB3102810 is still working at this time. Feedback on your experience would be useful on this.

If Windows Update is working properly, it should take 7 minutes maximum to find all 200+ updates after a fresh install of Windows 7SP1 on a SSD (From a clean Digital River or other Microsoft sourced ISO, in an activated or unactivated state, it doesn't matter)

It seems Microsoft don't won't you imaging up your system prior to installing Windows 10 and activating, then restoring Windows 7SP1 (i.e. to take advantage of the free Win10 offer, but carry on using Windows 7 for now). They want it to be a 'one way path'.

If you are thinking of upgrading to Windows 10, its actually better to do this from a fresh install, if you intend to carry on using your current Windows 7/8 installation or potentially go back to it. i.e. using your current install, to update to Windows10, the DNA of your current Win7 install seems to get recorded in the Microsoft Cloud Services, so if you then revert to it (outside of using MS Win10 tools, which reverts by deactivating Win10) by restoring a previous image manually, Windows Update is confused, as the version of Windows 7SP1 'shouldn't exist'.

Restoring Disk Images
If you do get a problem after restoring a disk image (Paragon, Acronis etc) of Windows 7SP1 and it starts endlessly checking for updates again, uninstall KB3102810 (assuming the backup had it installed) and then reinstall it. (but only if you get this, you shouldn't)

Just to add:
The above is only important if you intend to use the 'Reserve your copy' of Windows 10, so need to fully update Windows 7SP1 to 'current' or want to manually install KB Patches to date to avoid the Windows 10 Nagware and Telemetry patches, so you can just run Windows 7SP1, going forward, and not upgrade to Windows 10 (or be nagged about it)

If you intend to go directly to Windows 10, just download the latest build 1511 ISO, do a clean install of Windows 10 over the top of a clean install of Windows 7SP1 without even running Win7's Windows Update. This way you can avoid Win7's Windows Update all together and avoid the above problem.

The minimum requirement for installing Windows 10 from downloaded ISO/USB install is a fresh install of Win7SP1.

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They have Windows 7 Pro for £13.98, which would equally upgrade to Windows 10 wouldn't it?

Or even better stay on 8.1

These cheap OEM licences are being sold in breach of the OEM licence as they are not meant for re-sale. Microsoft have been known to revoke the licence keys. You are then left to try and get your money back from the seller. You might still feel its worth a punt but just be aware its not 100% lagit

Cheap for OEM Windows, must be clearing the shelves, well the virtual shelves.

Or even better stay on 8.1

can you just forego the upgrade with the previous version license?
when i upgraded the keys were very different between 8 and 10.

They have Windows 7 Pro for £13.98, which would equally upgrade to Windows 10 wouldn't it?

g2a etc have win 10 at circa 20 quid, saves all the messing about with immediate os upgrades etc. I'm my view that's better.

Is there a date on which the free upgrade to 10 ends?

Both upgrade to Windows 10 Home and not 10 pro?

Pro versions of 7 & 8 upgrade to 10 Pro