Which country has a flexible Constitution?

Concept — flexible versus rigid constitutions. A constitution is called flexible when it can be amended by the ordinary law-making process: the same legislature, acting by the same simple majority and following the same procedure it uses for any other statute. It is called rigid when amendment is reserved to a special procedure — a supermajority, a separate constituent body, or ratification by the federating units — so that constitutional change is deliberately made harder than ordinary legislation. The test is therefore procedural, not a matter of how long or how modern the text is.

Application to this question. The constitutional arrangements of the United Kingdom are not gathered into one codified document; they sit in ordinary statutes such as the Bill of Rights 1689, the Parliament Acts 1911 and 1949 and the Human Rights Act 1998, alongside binding conventions and common-law rules. Because those statutes are ordinary Acts, Parliament can alter any of them by a simple majority in the normal legislative way, and no separate amending procedure exists. That satisfies the definition of a flexible constitution.

Contrast — each of the other countries reserves a special amending procedure:

• India: under Article 368 an amendment needs a majority of the total membership of each House of Parliament together with a two-thirds majority of the members present and voting, and provisions touching the federal structure additionally need ratification by the legislatures of half the states. (Indian practice is often described as partly flexible and partly rigid, because a few provisions can be altered by simple majority under Articles 4 and 169, but the text as a whole is protected by Article 368.)

• United States of America: under Article V an amendment must be proposed by two-thirds of both Houses of Congress, or by a convention called by two-thirds of the state legislatures, and must then be ratified by three-fourths of the states.

• China: the Constitution of 1982 can be amended only on a proposal by the Standing Committee of the National People's Congress or by one-fifth of its deputies, and the amendment must carry a two-thirds vote of all deputies to the Congress.

Cross-check — apply the test in reverse. Ask whether the country's constitutional text can be changed by the same majority that passes an ordinary law. For India, the United States and China the answer is no: each demands a supermajority, and two of them also demand the consent of the federating units. For the United Kingdom the answer is yes. The flexible Constitution among the options is therefore that of the United Kingdom.