Veganism is a protected belief under the European Convention on Human Rights, as mentioned here in The (UK) Vegan Society's key facts page
Veganism is protected as a human right under Article 9 of the European Convention for the Protection of Human Rights.
This does mean, I believe, that prisons, hospitals and schools, for example, need to provide vegan options if they are requested.
Here is the treaty and here is the information on when each state signed and ratified it. It applies across the EU, in my understanding.
I suppose we are waiting for an update as to its future applicability in the UK :(
Article 9 – Freedom of thought, conscience and religion
Everyone has the right to freedom of thought, conscience and religion; this right includes freedom to change his religion or belief and freedom, either alone or in community with others and in public or private, to manifest his religion or belief, in worship, teaching, practice and observance.
Freedom to manifest one's religion or beliefs shall be subject only to such limitations as are prescribed by law and are necessary in a democratic society in the interests of public safety, for the protection of public order, health or morals, or for the protection of the rights and freedoms of others.