You can lose weight by eating a healthy and balanced diet that consists mainly of whole foods (vegetables, fruits, legumes, whole grains, nuts, seeds). Be sure to eat a lot of vegetables, including some leafy greens. You can easily find these foods in supermarkets in the UK.
If you want long term weight loss then why would you care about speed? Specially because fast weight loss is usually a problem (it’s kinda like malnutrition).
Usually the best long term diet for health and weight management is Mediterranean diet