British Politics and the British Constitution have many political conventions attached to it which have developed and been accepted over the years.
A convention is an accepted way in which things are done. They are not written down in law but tend to be old, established practices - the way they have always been done. Though these conventions are not set in legal stone, their very existence over the years has invariably lead to the smooth operation of government. This, again, is an argument for an unwritten, uncodified constitution. If the ways of governmental working were set in stone and had been for years (through a codified constitution), could government evolve and develop and mould to society's change if the way government works was rigidly stated in a written constitution?
There have been times when conventions have been given legal status. From the time of the English Civil War when Parliament clashed with king over finance, it was accepted that money bills/acts came from the House of Commons. This was given legal status in 1911 with the Parliament Act that stated that parliamentary finance bills/acts must originate from the House of Commons.
Some interesting conventions:
It is accepted that a departmental minister will resign if he/she loses the confidence of the House of Commons (i.e. within their own party). Usually, pressure is put on the person concerned to resign, as a sacking looks bad. In recent weeks, pressure from non-governmental sources (though primarily the media) has tried to tarnish the name of Stephen Byers, the Transport Minister, claiming that he has lost the confidence of the House. However, the simple fact is that Byers has kept his place simply because there has been no public party revolt against him - hence, why he has not resigned. A future Cabinet reshuffle would spare blushes all round as he was appointed by Tony Blair and it would look bad if Blair had to sack him if he did not resign.
It is a convention that the queen will accept the legislation passed by the government. In the past, the fear of what happened to Charles I has usually ensured a harmonious relationship between monarch and Parliament! When Charles II became king in 1660, the rule of thumb was for Parliament to give the king enough money per year to maintain a royal lifestyle but for him not to get involved in politics. This worked tolerably well and monarchs and Parliament had usually worked well since then especially as Parliament held the monarch's purse. Now in the C21st, it is just accepted that the queen will give parliamentary legislation the Royal Assent. It is almost beyond belief that she would not do so - the constitutional crisis this would create would be huge. The public backlash against an unelected person rejecting what a democratically elected government has pushed through would almost certainly be massive.
It is a convention that if something in government goes wrong, the cabinet will all sing the same song and support the minister who may be receiving all manner of criticism from the media. This has been very apparent with the recent history of Stephen Byers - all his cabinet colleagues have leapt to his defence over the problems he has recently faced.
Conventions can be changed, as they have no legal status. But they tend to be tolerated as they allow the system to work. Any reform to a convention has one major problem: how do you know that it will work as well as before it was reformed? It is does not work as well, do you admit defeat and go back or stubbornly proceed and make out all is well?