There is no constitution in the UK.
lol yes, there is, and they can call a constitutional convention any time, same as we can. They rely on traditions and several documents.
AI Overview

The UK's constitution is not found in a single written document but is a combination of written laws, like the Magna Carta, Bill of Rights, and the Human Rights Act, and unwritten constitutional conventions that have developed over centuries. This "unwritten" system has evolved organically, incorporating statutes, conventions, and landmark court decisions, and includes elements such as the powers of Parliament, the monarchy, and the judiciary, as well as established unwritten rules governing the relationship between these branches of government