He decides the date of the next election, any time between now and June 2006. There will be local elections next suummer, and recently most PMs have chosen to hold the general election on the same day. If he's behind in opinion polls, he'd delay until nearer June 2006, presumably. Currently it looks as though he'd win a small majority (
http://www.financialcalculus.co.uk/election/index.html), but most recent polls have indicated the Labour vote would be significantly higher if Gordon Brown, the current finance minister, were in charge instead of him.
The Labour party can in theory get rid of him as their leader, and thus the Prime Minister, with a special vote, but that seems very, very unlikely. Only slightly more likely would be a revolt by enough Labour MPs (about 80) in a vote of no confidence. That would either force him to resign, or, more likely, hold a general election immediately.