The High Court has given general guidance on overseas transfers of children for treatment particularly to EU member states (which may need to be re-considered if the UK leaves the EU).
The Courts have previously held that preventing a person from receiving treatment in another EU Member State constituted a restriction on the freedom to receive services under Article 56 of the Treaty of the European Union. Such a restriction had to be justified by member state (or agency of the member state - such as an NHS hospital or body) imposing it, for example on public policy grounds, if it was to be lawful.
When faced with a request by parents of an EU (i.e. including UK) child for transfer for medical treatment in another EU member state, the NHS Trust treating the child would have to consider the child's directly effective rights to receive services in other EU countries.
Where, having properly considered those rights, the Trust considered that a transfer would not be in the child's best interests (and the dispute was irreconcilable), an application to the High Court would be required to determine the resulting dispute as to the child's best interests. In those circumstances, it is highly likely that a refusal to transfer pending determination of such an application would constitute a justified derogation from their rights to receive services elsewhere on public policy grounds (and equally an eventual conclusion about what is in the child's best interests by the Courts would be a justified limitation of their rights).
The gold standard for determination of all matters remains what is in the specific child's best interests.
The guidance is given at the conclusion of the judgment in Tafida Raqeeb (by her litigationfriend XX) v Barts NHS Foundation Trust & others  EWHC 2531 (admin).
In this sad case, the TR had suffered a brain injury and could feel no pain and was unaware of their situation. The determination as to whether a transfer overseas was in her best interests therefore particularly needed to take account of ethical, moral and religious factors extrinsic to the child, such as futility, dignity, the meaning of life and the principle of the sanctity of life.