You shouldn't buy Windows 10 at launch unless you want it immediately. Most Windows 8.1 and Windows 7 users will probably get it for free within days of launch (remember to reserve it) and it looks increasingly likely that Microsoft will not crack down on Windows licenses whose origins are dubious (obtained either through MSDN, volume licensing or upgrading from a second hand machine).
The cheapest, totally legal route to Windows 10 is through Windows 7; Windows 8.1 being historically more expensive than its predecessor.
The cheapest Windows 7 currently on offer in the UK is from Ebuyer where Windows 7, either 32-bit or 64-bit, costs £62.95 with you key in the voucher code WINDOWS5 at checkout.
The offer ends midnight tonight but this is the second time in a month that Ebuyer has issued this offer so they may well get it soon again. The suggested retail price of Windows 10 stands at £99.99 so that represents a not-so-insignificant 37% saving.
Microsoft turning a bling eye?
There are more convoluted ways of getting it for cheaper, one of which involves eBay; you can either buy the operating system from a trusted seller with a good track record of delivering genuine software (one vendor had Windows 7 Pro on offer for under £30) or you can get a piece of scrap/laptop with a Windows 7 COA/Certificate of Authenticity attached (one vendor had sold more than 150 scrap PCs or laptops at £14 a pop).
Neither eBay nor Microsoft has cracked down on the above vendors despite them apparently openly flouting terms and conditions when it comes to transferring.
Microsoft clearly stipulates that "the OEM software is licensed with the computer system on which it was originally installed and is tied to that original machine. OEM licenses are single-use licenses that cannot be installed on more than one computer system, even if the original machine is no longer in use."
The TLDR version is, if a computer has a sticker, that sticker can't be moved or sold, as the license can't be transferred. It appears however, that for now at least, Microsoft is turning a blind (pragmatic) eye as all means, even the grey ones, are considered in order to grow the footprint of Windows 10.
If those moral quandaries make you uneasy, nothing prevents you from buying a relatively cheap second hand PC (many available from less than £40 from eBay) with the only purpose of scavenging the device's COA for use elsewhere. There is next to no chance that Microsoft will ever block the license.
Bear in mind that if you are not buying a genuine Windows 7 copy, there's always a (tiny) risk that things may turn sour in the future.