constitution is flexible

 The UK constitution consists not only of statutes, but of conventions, practices and underlying principles such as parliamentary sovereignty and the rule of law which means it can be changed in a number of ways. 

The UK Constitution can be amended (changed) by passing laws (statutes) though Parliaments- It's no harder to change the constitution than to pass any law.

Examples include the following:

Conventions develop in ways which can amend the constitution

Unwritten practices, usually known as conventions, develop over time. These are unwritten rules to which everyone in the political system adheres. Thus, over the past 70 years, we have seen a number of changes as a result of new conventions. Examples include the following:

Referendums that confirm amendments to the constitution

Referendums are increasingly used to confirm constitutional change. Examples include the following:

Courts can change the constitution by interpreting the law which establishes new precedents and even new rights