There is no constitutional reason for a UK prime minister, who takes over the job in between elections, to call for his own personal mandate. The reason is that the UK electorate vote for a party, a team of individuals that will run the country, in theory because that party has a collective vision, perhaps an ideology, that suggests how they will design policy and effect the lives of the citizens. The change in party leader should be a marginal event really, he is not an all powerful president but, theoretically anyway, the person that ensures the party is representing its electorate. If any readers see some disparity between the 'theory' and what they view as 'practice' then perhaps that is why there is a sense of scepticism, cynicism and abstention from voting.
However since Brown's coronation David Cameron led demands for an election and the media began to speculate about whether he wanted a personal mandate, and this speculation exacerbated as his popularity in the polls rose. There then seems to have been a cat and mouse game going on. Brown was forced to enter into speculation, any signs were used to indicate if there was an election or not, and the media pushed him for a response, he smiled coyly and kept them, the UK citizens and his opponents guessing. He was in the position of power and relished it.
Perhaps bravely, perhaps foolishly, Brown yesterday claimed responsibility for allowing the speculation to continue and tried to play down his reasoning for not having an election. The power then slipped to his opponents and those 'feral beasts of the media'. Well of course you cannot admit you wont call an election if you're afraid of losing, even though everyone is aware that is one of the key concerns of politicians within an era of permanent campaigning and fervent poll watching.
Interesting little fact on one report of the polls, and Brown's potential reasoning: "Sunday's News of the World suggested the Tories were ahead by 6% in marginal seats, with the party overall at 44% against Labour's 38%. Translated into a general election, it would mean a hung Parliament with Labour holding 306 seats and the Tories 246". No wonder Brown is confident of winning, all he needs is to reverse the polls slightly, draw even, and the likelihood is a narrow but workable majority. This in itself says little about the state of democracy and how motivated voters should be to make their choice.