Under common law, you can use reasonable force in such circumstances.
Also sect 3 of the Criminal Law Act will state that reasonble force can be used to save life and limb. Therefore a straight translation for this scenario would be if the "patient" is unfit through illness or injury to take on board treatment such force may be used.
Also under the Mental Health Act, force again can be reasonably used in certain circumstances.
If restraint is used to protect oneself from injury or to protect property, it will make no difference whether the person is hospitalized or not. The same will go for an arrest to be made. Though medical care outweighs arrest law. The doctor can therefore insist on the removal of handcuffs whilst under medical care.