Children do not have formal rights in English law. Although the UK is signatory to the UN Convention on the Rights of a Child this does not have the force of law in England & Wales.
The child’s relationship with parents and the state under Parental Responsibility acknowledges that children have some rights and that parents and the state have a duty or obligations to the child.
Gillick competent children are those 16 or younger who are deemed able to consent to medical treatment without parental permission or their parents knowledge. Even with Gillick competent children the courts are likely to intervene if the child is making a decision which threatens its life.