Anorexia is a psychological illness, even though its effects are mostly physical as the root cause of the issue is mental. For this reason, sufferers of anorexia could be sanctioned under the mental health act as unable to tell the difference between what is good and what is not for their bodies. If someone is sanctioned under the mental health act then the health care workers in charge of their care can legally provide them with any medical care that they feel is in the best interests of the patient, including force-feeding. Thus the force-feeding of anorexia sufferers is completely legal.
No, it is not legal to force-feed anorexia patients, because anorexic people have a right to make their own decisions about their bodies, even if their decisions ultimately kill them. We allow smokers to continue to smoke, knowing that it will kill some of them. In the same way, an anorexic should be allowed to refuse to eat, even knowing that it might kill them. A person has a right to control their own body, even if they make poor choices.
Just like any other adult, anorexics should have the right to make their own lifestyle choices and medical choices, including not eating and denying medical care. While it is not advisable, many people do things that are not good for them and as long as they don't harm anyone else, nobody has the right to interfere. Force feeding anorexia patients is a form of interference in personal choice that should be illegal since anorexic behavior does not harm anyone else.