Even in the age of the internet, the national political agenda is still largely set in the UK by a small group of editors – those of national newspapers and the broadcasters – plus people in the public relations industry on behalf of their clients, including politicians.
The national political agenda largely governs what the politicians, the papers and the broadcasters talk about each day. If a subject is on the agenda, it is more likely that there will be effective political action. Conversely, if an item is not on the agenda, it will probably not be discussed and it is less likely that there will be any effective political action.
There are plenty of issues of importance to millions that rarely, if ever, are on the national political agenda. A good example is poverty and destitution in the UK. This is the central issue in the lives of millions. However, as it is rarely on the agenda, the issue can be easily ignored by politicians.