You can commit an offence under Section 14 of the Food Safety Act 1990 if you sell something which is not all of the substance demanded by the purchaser. In practice this is likely to apply if a customer is given inaccurate information in a description of the food.
The presence of some ingredients may not be obvious to customers from the product description. If a recipe includes an allergen such as nuts, make it really obvious by mentioning them on the menu. For example: “Strawberry tart with almonds.”