ANd as an aside. All legal copies of windows are liekly to be pretty much the exact same price depending on version in any particular market (I.e. the UK). The only real price difference you might see (of more than like 10 pounds/$$) is between OEM and Retail. OEM is a good bit cheaper, but once you install it on one PC you can;t then later on transfer it to a newer PC. With Retail you can.
But if by then you expect to use Windows 8 on any new PC, you can save some money buying an OEM version along with the hardware you buy (look on the site/store you are buying the motherboard and or hard drive from, they may have OEM copies for sale and you MUST buy the copy along with either a motherboard or a harddrive or some other major internal computer part).