The Constitution of the United Kingdom is the oldest and most peculiar of the three. It is not written in a document, it is "an uncodified constitution", so it is based on a set of customs, usages and codes that establish parliamentary sovereignty, democracy and the rule of law (including the upholding of international law and human rights). The British constitution is interpreted by the UK Supreme Court, and it applies to the United Kingdom, including Scotland, Wales and England after the Acts of Union (1707) and to Northern Ireland after the Acts of Union (1800), which were signed by the kingdoms of Great Britain and Ireland. The UK Constitution dates back to the Magna Carta signed by King John of England (1215), which limited the absolute power of the monarch and granted some basic legal rights. The Magna Carta was later on augmented by the Bill of Rights (1689), which is a crucial statute in English constitutional law and sets out more civil rights like freedom of speech; the right of petition; the right to suffer no cruel or unusual punishments; legal immunity for members of Parliament (on actions of deeds made in the course of their legislative duties) etc.
In recent years there has been talk and some initiatives to reform the UK constitution. Devolution of power to the Scottish, Welsh and Northern Ireland assemblies, has been the most conspicuous. Here you can read the BBC article "Devolution: What is it and how does it work across the UK?", which comes with a C1 lesson plan. At the bottom of this post you will find some of the key words in this article.The Institute for Government, a moderate centre-left think tank, and the Bennett Institute for Public Policy at the University of Cambridge have recently published some reports to review certain constitutional issues, like Parliament, devolution in England, electoral reform, the House of Lords, the Monarchy, the role of the political parties etc. The video "Review of the UK Constitution" (6:48) summarizes their goals and conclusions. The use technical legal vocabulary (jargon), and the density of complex constitutional concepts in the narration, raise the difficulty of the video and make it suitable for C2 students, even though there are subtitles. If you are extremely interested in the subject, you can check out the Institute for Government web page, and the short conclusions in the Review of the UK Constitution: Final Report, which is recommended for Experts, (above C2 level). Brian Christopher Jones, on the other hand, offers arguments to keep the unwritten UK constitution as it is now in the article A single written UK Constitution may only make things worse. This highbrow academic essay is suitable for experts in British law (Ex level).
The Spanish Constitution (1978) is the supreme law of the Kingdom of Spain. It is the culmination of the Spanish transition to democracy, and its approval in a referendum meant the legal end of the the Francoist dictatorship. According to Wikipedia, "article 1.1 states that Spain is a social and democratic state under the rule of law with the legal values of liberty, justice, equality and political pluralism. Article 1.2 refers to national sovereignty, which is vested in the Spanish people, article 1.3 establishes parliamentary monarchy as the political form of the Spanish state". All in all, there are 169 articles and a number of additional and transitory provisions in the Spanish Constitution.
Some of the key words in the body of this post are: codes, sovereignty, to uphold, to date back to, to grant [legal rights], to augment, a statute, to set out, a petition, deeds, devolution, conspicuous, a think tank, jargon, highbrow, [the three] branches [of power], the cabinet, to be amended, civil rights, gerrymandering, incarceration, to overturn, a court ruling, a dictatorship, to state, to be vested, a provision.
Some of the key words in the BBC article "Devolution: What is it and how does it work across the UK?", are: to be run, an elected body, policy, tax, to raise/ lower [income tax], forestry, courts, stamp duty, welfare, to argue for, highways, landfill [tax], power-sharing, mandatory, to be dissolved, [relations] break down, a mayor, to set [priorities], [bus] fares, the Tube, a target, affordable.
Some key words in the TED-Ed video, "U.S. Bill of Rights" below include: assembly, to bear [arms], an amendment, search, seizure, privacy, a trial, a jury.