The UK is said to have an unwritten constitution. However, it would
be better described as uncodified, as most of the constitution is laid
out in writing such as constiutional statutes, but there is not one single
document labelled the constitution.
A written constitution is an attempt by politicians and
statesmen to codify all the important laws and rules
relating to the way the state will be governed.
America and France are said to have influential
constitutions.
Rigid and Flexible
A rigid constitution can offer the people of a country stability.
When changes are needed, certain procedures must be followed
to be able to change the constitution, ie. the gun law USA.
A flexible constitution such as the UK's can be changed fairly easily, ie.
by passing an act of parliament. Sometimes, big constitutional decisions
may open up the doors to referendum, however in a flexible constitution,
this is not a constitutional requirement and there is no obligation to hold
one.