A third of Britons are overweight, states a report published in January by the Royal College of Physicians, the result of an 18-month study. About five per cent of children weigh too much, and are likely to stay that way for life; in the mid-twenties-age-long group the proportion  of the middle-aged population half are overweight.     Fat people risk severe health problems, says the report, including high blood pressure, breathlessness and various forms of heart disease. Smoking is particularly risky for overweight people.     The safest way to lose weight is to eat cereals, bread, fruit and vegetables, and cut down on fatty meals, butter and sweet foods. Fad diets so far more than good; slimming machines that vibrate muscles have not been proved useful; saunas merely remove a little body water, and health farms, says the report ,serve as expensive holidays.     Exercise is most important to health, the report emphasizes; though it doesn't necessarily reduce weight; it keeps the correct proportion of body muscle. And it isn't only for the young ; from middle age a minimum of 20 minutes of gentle physical jerks should be practiced three times a week.     The report advocates several public health measures to fight the common existence of overweight in this country. They include an increase of tax on alcohol to reduce its increasing, and dangerously fattening, consumption; and the provision of more sports facilities by local authorities. Britain's doctors, the report concludes, must learn to be more sympathetic and specific in their advice to the overweight, encouraging a change in eating habits on a long-term basis and taking into account the many-often-complex-reasons why fat people are fat. The report points out that drinking too much alcohol  _  .
will also cause one to get fat will cause one to do less sports will make one forget the fact that he is fat. will lead one to bad eating habits