From the Food Standards Agency
"Use-by dates are about safety
A use-by date on food is about safety. This is the most important date to remember.
Foods can be eaten until the use-by date but not after. You will see use-by dates on food that goes off quickly, such as meat products or ready-to-eat salads.
For the use-by date to be a valid guide, you must carefully follow storage instructions. For example, if the instructions on the packaging tell you to refrigerate after opening, you should keep the food in a fridge at 5°C or below.
Find out more about chilling your food correctly.
After the use-by date, don't eat it, cook it or freeze it. The food could be unsafe to eat or drink, even if it has been stored correctly and looks and smells fine. A lot of foods, including meat and milk, can be frozen before the use-by date though so plan ahead."