They will both make beer, but the stuff you get from a home brew supply co will probably make a beer more like one intended in the recipe you are following. I'm not sure what they sell at Sainsburys, but at my LHBS I can get malt extract in extra light, light, amber, dark, etc, and while they will all make beer (just add hops, water, and yeast...), they make very different beers. Most of the recipes that I brew use extra light extract, and if I used a dark extract instead it would really mess up the profile of the beer. Make sense?