A six-month behavioural intervention reduced ultra-processed food consumption by 25% of total energy intake among 45 UK adults with overweight or obesity. Participants simultaneously increased minimally processed food intake by 23% and vigorous physical activity by 60 minutes weekly, while losing an average of 3.8 kg. The intervention achieved 91% uptake and 68% retention at six months, with high fidelity to behaviour change techniques. This multi-component approach represents a promising step forward in translating growing research on ultra-processed foods into practical interventions. Unlike previous studies that often focus solely on calorie restriction, this program specifically targeted food processing levels—a distinction that may prove crucial given emerging evidence linking ultra-processed foods to metabolic dysfunction beyond their caloric content. The theory-informed design using established behaviour change frameworks strengthens its potential scalability. However, the single-arm design without a control group limits causal conclusions, and the relatively small sample size calls for larger trials. As a preprint awaiting peer review, these results require validation through controlled studies. If confirmed, this approach could inform public health strategies addressing the UK's rising consumption of ultra-processed foods.