When health charities let their logos appear on food products, they could be doing more for company profits than for public health, according to a new survey by the Food Commission to be published tomorrow [Tuesday].   Logos or endorsements from health charities and medical associations appear on brands of fruit juice, bread, tomatoes, cooking oil, margarine, porridge oats, milk, yogurt and even tea bags. National organisations that allow their logos to be put onto food products include the British Heart Foundation, Family Heart Association, Cancer Research Campaign, the National Osteoporosis Society, the British Dietetic Association and the British Dental Association.