There are a number of ways you can find a qualified professional to advise you about your diet. You could consult either a dietitian or a nutritionist, depending on the kind of advice you want.
Registered dietitians (RDs) are experts in diet and nutrition, so they can provide advice on all aspects of eating and diet, including special diets for medical conditions. The title 'dietitian' is protected by the Health Professionals Council (HPC). This means that someone can't call themselves a dietitian unless they are suitably qualified and registered with the HPC. Registered dietitians are regulated by the Government. The professional association for dietitians is the British Dietetic Association.
You can contact a registered dietitian through your local hospital or GP surgery. You can also search online for a dietitian at Dietitians Unlimited, which is run by the Freelance Dietitians Group of the British Dietetic Association. The HPC publishes its register online, so you can check on its website to see if a dietitian is registered.
Nutritionists are qualified to provide information about food and healthy eating, but not about special therapeutic diets. The Nutrition Society has a register of nutritionists (R Nutr), associate nutritionists (A Nutr) and public health nutritionists (R PH Nutr). The purpose of this register is to provide a list of nutritionists who have received an approved level of training.
A list of registered nutritionists can be found on the Nutrition Society website.