From Friday 1 October 2021 foods which are classed as pre packed for direct sale (PPDS) will be required to be labelled with information about the allergens they contain.
PPDS food is food that is packaged at the same place it is offered or sold to consumers. It is a single item, consisting of the food and its packaging, that is ready for presentation to the consumer before it is ordered or selected. The Food Standards Agency web site has a decision tool to help determine if your food is PPDS - Allergen and ingredients food labelling decision tool.
Any food business that produces PPDS food will be required to label it with the name of the food and a full ingredients list. Allergenic ingredients must be emphasised within this list.
There is lots of detailed guidance on the requirements on the Food Standards Agency website - Allergen labelling changes for prepacked for direct sale (PPDS) food.