A vote of no confidence would do it.
The following is taken from 'thinkquest.org'
Parliament can also be dissolved if the government is defeated on an important piece of legislation. When a Parliamentary majority votes against the legislation it is treated as a vote of no confidence for the prime minister and his government. A specific vote by that name may be taken to indicate that the majority of MPs are against the legislation. This tradition is so deep that actual votes of no confidence are rarely taken. The government of Labour Prime Minister James Callaghan was dissolved in 1979 when a vote of no confidence was taken after union workers went on strike in reaction to the government's attempt to limit wage increases. There had been no such vote of no confidence in Britain since 1924.
However, I think this is unlikely at the moment, but don't quote me on that ;-)