There are no nationally prescribed criteria for entitlement to see a nutritionist on the national health service, rather it is a matter for each individual health practitioner to make a clinical decision to refer a patient if they think it advisable, in line with any locally determined guidelines that may apply.
The National Institute for Health and Clinical Excellence clinical guidelines on Nutrition Support in Adults covers the care of those patients with malnutrition or at risk of malnutrition at hospital and at home.