"In Manchester Withington, one person does not really have one vote, they have the equivalent of 0.851 votes. The power of voters in this constituency is based on the probability of the seat changing hands and its size. While you might think that every vote counts equally, where you live in the UK has a huge effect on your power to influence the election."
Check Votepower. A word of caution, aparthy can give results you never wanted.