There have been many different approaches to defining what a constitution actually is, however, for the purposes of this text, I will be using the definition given by the House of Lords Select Committee on the Constitution, 2001:
‘The set of laws, rules and practices that create the basic institutions of the
State, and it’s component and related parts, and stipulate the powers of
those institutions and the relationship between the different institutions and
between those institutions and the individual.’
To illustrate the point, it must be said that the vast majority of countries have a written constitution, videlicet, a single document which encompasses all of the above, for example the Constitution of the USA (within which is found the ‘Bill of Rights’). Evidence of this is to be found in the first three articles of the document which set out the roles and powers of (Art 1) the Congress, (Art 2) the President and (Art 3) the Supreme Court. The UK has no such constitution; that is to say we don’t have one in written form. Our constitution is established through many sources, the traditional ones being legislation, judicial decisions, and constitutional conventions.
Whether primary legislation (Acts of Parliament), or delegated (made by other authorities on whom Parliament has conferred the power to legislate), statute law plays a key role in sourcing constitutional and administrative law. For example, the Human Rights Act 1998 gave effect to the European Convention on Human Rights, and the Bill of Rights 1689 which provided restrictions on the Monarchs power without the consent of Parliament.
Judicial decisions, whether that be in the interpretation of statutes or in the formation or furtherance of common law, also have the capability to change the arrangement of our constitution. When interpreting statutes, in the absence of express words or implications by Parliament, the judiciary presume that Parliament does not, for example, intend to alter the existing rights and privileges of the Crown (Lord Advocate v. Dumbarton DC  2 AC 580), or give retrospective effect to penal enactments (Waddington v. Miah  I WLR 683).
Political conventions are perhaps the most unpredictable of the three methods of changing the constitution because, as is the norm with politics, public policy and public morals can change in unforeseeable ways at unforeseeable times. Dicey described political conventions as ‘constitutional morals,’ which validates that point.
There are only three countries in the world which have an unwritten constitution, or more specifically, un-codified, namely New Zealand, Israel and the United Kingdom (although New Zealand has recently taken steps towards a written constitution with the enactment of the Constitution Act 1996). In his book ‘Constitutional and Administrative Law,’ Alex Carroll describes the UK constitution as ‘a myriad of judicial decisions, Acts of Parliament and established political practices (conventions). It is in this sense, therefore, that the constitution may be defined as ‘unwritten’.’ He goes on to explain that the reason why our constitution has remained unwritten could be put down to the fact that we haven’t had ‘the sort of cathartic political events (e.g. defeat in war or civil insurrection) which, in many other countries, have led to the abandonment and replacement of a pre-existing constitutional order.’ What this means is that the reason why so many of the countries have written constitutions is because they have had periods of political upheaval, giving them the opportunity and the reason to reform and codify their ‘pre-existing constitutional orders.’ The last time the UK had such a major political disruption was in the seventeenth century, witnessing two major rebellions (or civil wars), both of which led to the demise of individual monarchs. Although this upheaval did not result in the adoption of a permanent and written constitution, it did lead to the creation of the most significant, post-revolutionary, statutes, namely the Bill of Rights 1689 and the Act of Settlement 1700, which remain to this day, fundamental elements in the UK constitution.
Due to the structure of our constitution, it is far easier to make changes to it than in other countries. For example, to make an amendment to any part of the constitution of the USA, it is required that a majority of two thirds of both houses of Congress, and the legislatures of three quarters of the several states, agree upon it (Art 5). This method of protecting constitutional fundamentals is called ‘entrenching.’ However, in the UK there are three ways to make amends. The first is through legislation. A new piece of legislation, passed according to normal Parliamentary procedure may change any part of the constitution. Judicial decisions in normal cases may also have an effect, making this the second method. Changes in existing conventional practices can also lead to a change in our constitution. Because there are three methods and they are each easily implemented, then it’s shows us that our constitution has the ability to be very flexible. This has been both criticised and praised. Those in support of the current, unwritten, format emphasise its ability to change quickly with changing times and this characteristic of flexibility. Those who are in favour of forming a written constitution argue that due to the executives’ majority in the House of Commons, it is possible that the government could, Alex Carroll says, ‘impose constitutional change purely for reasons of political expediency and ephemeral ideology.’
While it can be said that our constitution is open to abuse due to it’s extreme flexibility, there are examples of clauses which are immutable, for example the basic human rights requirements of the 1949 German Constitution. Calvert, in his book ‘An Introduction to British Constitutional Law,’ explains that the ideal format of a constitution would be one which avoids ‘the Scylla of total rigidity and the Charybidis of total flexibility.’
As we can see, our constitution is not protected against corruption to the same level as other countries and this is what worries those in favour of a written constitution.