The country without a constitution....
The UK does not have a constitution per se, certainly not in the same way that the US or Germany does. Instead we have a series of rules, guidelines, powers and laws which govern how the country is run and where power lies.
This area of law makes sure that that power is not abused and that it is used in an appropriate manner. Someone has to keep a check on the government and that is where this area of law steps up to the plate.
Why is it important? What does it involve?
Constitutional or administrative law is arguably the most important in existence; without it there could be no other laws and the state would not be able to function. It holds our country together.
Practising administrative or constitutional law means you will either be employed by the state in the Government Legal Service (GLS), or in a private practice. Incidentally, the GLS is the largest employer of lawyers in the UK.
As a lawyer in this area, you will be working to ensure that government bodies and agencies are acting in the public interest. Therefore, you could be working alongside the NHS, your local council or the police. Sometimes the state does not follow the interests of the public, or works in contravention of its obligations under national and international law. For example, if a local councillor has ten parking permits and gives them all to their friends and family for no other reason than because they are related to them, then this is clearly unfair and administrative law can be applied to make sure they does not abuse their position.
Similarly, if the police torture you whilst you are in custody, then there will have been a breach of the European Convention of Human Rights (Art. 3) and your action against the state will be pursued through this constitutional or administrative law.
Break it down for me a little bit!
If you would like to work in the public interest, either for the state or in a private practice against the state, constitutional and administrative law is a stimulating and exciting option.
You may be dealing with high-profile cases or smaller local issues, but either way you will have to deal with lots of paper work (the public sector is notorious for its bureaucracy/paper work) and keep up-to-date with constantly evolving case law.