Coopers Beer Kits contain hopped liquid malt extract. Liquid malt extract darkens over time and develops toffee/molasses like aromas. This process is accelerated when temperature is increased. This is not such a bad thing with intentionally dark brews but may be undesirable with lighter styles. If you wish to persist with making up a brew past its Best Before date (out of curiosity or otherwise) it’s worth getting fresh yeast from a Brewing Specialist store to ensure that the brew ferments thoroughly.