Declination correction. to account for magnetic variation.. 'East is least, West is Best' (subtract easterly variation and add westerly)... in England, all variation will be westerly, probably about 5 degrees, but that's entirely dependent on several factors. A good topographic map will indicate current variation data...
The Agonic line (0 degrees variation) is a non-linear line of zero variation that runs around the world (N-S). It runs roughly through the State of Indiana here in the U.S.and over the Island of Jakarta, Indonesia on the other side of the globe... Here's a chart to show you how big the numbers can get over the surface of the planet:
http://www.mission-planning.com/Jan05_News.htm
Best of luck!