I don't know what the rules are in the UK, but in the States the only foods with rules about dates are baby food.

Other foods are entirely up to the manufacturer. I had a friend who worked for Nabisco, whose duties at one point included doing spot checks on Nabisco products in grocery stores for date compliance. She quickly learned that the dates made no sense compared to her experience in when foods actually went bad. Often the "best by" date is an arbitrary guess at when a food starts to decline on look or flavor (NOT edibility); the "sell by" dates are guesses as to when a product will be less shelf-attractive than its competing neighbors.

The worst example was dog biscuits — the date was not because the treats went bad (apparently dog biscuits could be put in an Egyptian tomb and still be good 2,000 years later) but because there were fears that boxes left on the grocery shelf too long would get dusty and less attractive.

So in the US, at least, go by your nose and your eyes, not the dates.