The advantage of not having written or codified is that laws and policies are evolutionary and can be easily amended. An example of this is the in-corporation of E.U laws into UK laws. Since parliament passed the European communities Act in 1972 Britain has accepted the superiority of European law. The House of Lords has judged certain English laws to be unlawful in light of EU legislation.
Another advantage of an unwritten or un codified constitution is that it is evolutionary because it develops with historical changes. An example of this is when parliament in the UK took total sovereignty away from the monarchy in 1867. Changes to the British constitution reflect a changing balance of power. The constitution is important even though it is not written form because it formed the basis of the separation of power that we now have, for example between the Lords and the Commons. This also shows the flexibility of the constitution.
An advantage of the UK constitution is that it takes into account of changing views. For example, in 1997 the changing of the hereditary peer system and also further reforms to change the structure of parliament. It is good that the British constitution will always takes into account these changes.
However, there are disadvantages to an un-written constitution. A written constitution is considered the highest form of law and is respected because of this. There is no such principle in the British constitution because sovereignty lies with