The unwritten rule is that in order to be given a licence to broadcast then those people given the contracts must play by the governments rules or face having their licence withdrawn. In this manner the mainstream stations have to take what is erronesously called a 'moderate' position. I believe the term is wrong because I don't believe anyone should be bombing anyone and therefore someone who wants to mount a defence for bombing others (or trashing wildlife, or releasing toxins into the air etc etc) has a considerably less moderate view than I.
In the U.K. other broadcasters are forced to compete with the BBC for viewers (or listeners) and if the BBC has found a popular role (which it has) in taking the supposedly centre ground (which in reality is nothing of the sort) then in order to achieve maximum profit by capturing the largest audiences the larger commercial organisations will have to take the centre (establishment) ground too.
The Guardian Media Group (GMG) have complained that this power was also used to lever the BBC into a favourable position in the newer broadcasting arenas such as the internet (BBC online) and digital broadcasting (BBC 3 and 4 for example). They also stress that the effect of this is not to be underestimated.
"The entry of the BBC into a market can render commercial alternatives not viable, deter new entry or commercial innovation, or lead to consolidation, or concentration which could serve to reduce competition, diversity and choice."- Advertisement -
What the GMG suggested is that contrary to the rhetoric, the BBC, in striving to create an inclusive and diverse broadcasting service has become such a monolith of British broadcasting that other interests are regularly forced out of business (or do worse than they could have otherwise expected) due to its influence. This, if true, does little to enhance the plurality of voices available for the public at large to hear. In fact, it leads to the "Dominant Ideology" theory as put forward by the Glasgow University Media Group. At its most basic level this theory suggests that a publicly owned media in a democratic state will enforce a kind of uniformity on the media.
" The BBC is a national institution in so far as it consistently promotes the illusion of a unified and integrated system of common values and beliefs. It's very existence perpetuates this myth"
The BBC is supposed to be editorially independent. However, it has an institutional goal of being an authoritative 'Voice of Britain' figure. Whether this goal was self-realised or not is another matter. The point is it has become self-perpetuating.
It must therefore, not be too critical of the state as, in spite of its supposed editorial independence, the success of the institution that is the BBC depends on the success of the institutions of the British Government. The BBC depends on the government for funding - basically for its life. An unhappy government may refuse to increase the licence fee and choose to suppress the BBC in this manner so the BBC cannot be too disparaging of the British state. After the Hutton inquiry, a BBC journalist said words to the effect that investigative reporting at the BBC is not dead but investigative reporting into the British Government is dead.
Its investigative programmes are never massively damning to the system as a whole, merely to individual errors or policies. This BBC maintains the British regime by examining the individual errors within the context of the British political system whilst never examining the foundations of it. Certain things usually go unreported such as the fact that the Queen Mother once had Oswald Mosley, the leader of the British Fascists round to tea.
Individual politicians or policies may be criticised but the system underpinning them is rarely called in to question in mainstream broadcasting. For commercial channels this may be due to considerations of profit (political programming not being particularly popular). Or it may be due to a wish to have a licence renewed. Or indeed it may be because we have a wonderful fully comprehensive system that requires no further attention.
Private and public media do it for different reasons. This media system makes for profits and creates an illusion of national unity, it does not do it in the interests of free speech.
 I should really stop trying to be satirical. "Political satire became obsolete the day that Henry Kissinger won the Nobel Peace Prize". - Tom Lehrer.