Where do you live? I'm in the US and use US sources; sounds like you are in England. I'll provide a link to the wonderful list of international sources.
But there are really four possibilities. In increasing order of cost: Black market (cracked, torrented, whatever) which are illegal and likely pre-loaded with malware.
Grey market, like an OEM disk provided to or by a manufacturer of PCs that may or may not work with your hardware. Sometimes it's hard to tell grey from white on eBay. There are perfectly good unused licenses there, but I wouldn't go for anything other than Full Retail in original shrink wrap.
White market, here in the US old stock at Staples, Microcenter, or the ever-reliable Newegg.com, but now we're getting up the price ladder to over $100 US.
At this level what are labeled "OEM" versions from Microsoft are likely legit - meant for manufacturers, but allowed for home builders. The OEM license can only be used on one system; if you build a new one an OEM license can't be transferred and a full retail can. BTW, Windows 8 licensing is simpler.
Gold market: Buy it from Microsoft. For all I know, they have discontinued sales of W7.
Here's the list; UK is fairly far down. http://www.tomshardware.com/forum/id-1643019/guide-international-users-links-part-stores.html