A recent article of the Committee of Advertising Practice (or CAP) comprehensively captures the rules applicable to foods and foods supplements which promote weight loss or related claims.
The CAP is a body responsible for the creation of rules aiming to guarantee proper conduct in the promotion of products and services through self-regulation of the advertisement industry. The conformity to the codes and guidelines it establishes are being monitored by the Advertising Standards Authority (ASA). The article provides reference to the applicable codes as well as relevant regulations regarding nutrition and health claims, with a focus on those related to weight loss. The article provides a detailed overview of the types of potential claims that exist using examples, including rulings adopted by the ASA, and the use of relevant links for further reference.
CAP Advice Online - Weight control: Food and Food Supplements - link