As I wrote earlier this week, the timing of British general elections is at the discretion of the prime minister. There is a five-year maximum interval between general elections (with very rare exceptions), but theoretically no minimum.
There are, however, practical reasons why a prime minister can't call for a new election too often. For one thing, the expense of holding the election could become a political issue. Even the much-smaller expense of holding a by-election in one constituency was an issue in a recent case, where the need for the by-election was questionable.
Also, if people are asked to vote too often, they can tend to exhibit a combination of irritation and apathy, often labeled "voter fatigue".
But in some cases it is advantageous for a prime minister to call an election much earlier than is usually the case. That is called a "snap election".
Perhaps the most successful such case was in 1966 when Prime Minister Harold Wilson decided to go to the voters less than a year and a half after winning the October 15, 1964, general election with an overall majority of four seats in the House of Commons. In the snap general election of March 31, 1966, Wilson's Labor Party increased its overall majority to 96.
In a much more recent case, however, a prime minister allowed speculation about a possible snap election to reach fever pitch, only to pull back at the last minute and postpone the election.
The current U.K. Prime Minister Gordon Brown proved popular in the opinion polls after he took over that office from fellow Labor Party member Tony Blair on June 27, 2007. By October of that year, Brown had everyone anticipating an imminent election. Then, the opposition Conservative Party gained in the polls, after the Conservatives' leader, David Cameron, made a well-received speech to his party's annual conference. On October 6, Brown announced that there would be no snap election.
Many commentators said that Brown came out of that sequence of events looking simultaneously weak, opportunistic and dishonest. One of the fictions of the process of setting a date for a U.K. general election is that the prime minster makes his or her decision solely on the basis of national interest. A prime minister cannot publicly admit to calling an election based on good opinion polls, or declining to call an election based on bad polls. It certainly looks as though Brown did the latter, but he can't say so.
Brown's Labor Party has sunk further in the polls since last October. Now, it looks as though he'll follow the usual pattern that I noted earlier for a party in such circumstances, and call the election in 2010, five years after the previous one.