CompareMagazines.co.uk is an online Magazine subscriptions website, where you can find the cheapest magazine subscription. The site compares and find the best prices offered by the most popular magazine's subscription providers in the UK.
They have an amazing list of magazines in their database - 1062 UK magazines when I last checked.
You can browse the magazines by category or by title or simply searching for it, if you know the title. For example, searching for Cosmo gives me 4 results. I was actually interested in checking out the Cosmopolitan subscription links. Prices given here were slightly different from that on the actual sites.
Another check for GQ subscription, called the the best men's magazine in the world, showed that there subscription rates can vary a bit (£1.20). However, Vogue subscription rates varied by much as £3 between the two subscription providers listed. However, the total prices come out much lower than prices at the newstands.