@ Jim, you vote for a local MP for your district. whichever candidate for your district gets most votes takes the win and represents your area in Parliament, he takes that "seat". whichever party takes the most seats in Parliament (hence most local councils across the country) becomes the party in power, and their leader (Cameron, Clegg, Brown or other) will become PM. we don't directly vote for either of those 3 party leaders to become PM, we merely choose which party should hold the power in our area, and then that knocks on to the country as a whole; it's for the local MPs who sit in Parliament to vote who their leader is and therefore who is the candiate for PM.
for a power to definitively win the election, they have to win by a certain majority, they can't win by 1 or 2 seats, they have to won by 100-odd or something. if the 3 main parties all win a similar number of seats with no clear winner with a big enough gap to 2nd place, it's a "hung parliament" where no winner is declared. the 3rd place party then has the choice of forming an alliance with one of the 2 parties ahead of it, thus making that party the winner.
so the Conservatives could win, Labour could be 2nd with 5 seats less, and the Lib Dems could be 5 seats less that Labour, so no winner would be declared. then whichever party the Lib Dems gave their backing to would win, so Brown would suck up to Clegg and promise to implement some of the policies the Lib Dems wanted to implement had they got into power, in exchage for Clegg's support to make Labour win the election.
there's a strong chance i'm wrong about a lot of this as i don't know the system properly, so please correct me if i am
RIP Jules, never to be forgotten. #KeepFightingMichael