The UK’s political system, as it currently stands, is a two party system. This has been in large part due to the First Past The Post (FPTP) system, in which only those voting for the winning party in their constituency are actually represented in parliament. If your favoured candidate doesn’t win, then your vote is essentially discarded.
Contrast this with proportional representation such as used for European Parliament elections, where constituencies are larger and elect a small batch of MEPs. In this system, the winner still gets the most seats, but the second and even third place parties are also rewarded in proportion to the votes they receive. If your favoured party doesn’t win, you don’t automatically lose your say on who gets elected – and more voters are actually represented in parliament.
As a result of First Past The Post, it’s common for the two largest parties to gain a disproportionately large number of seats in parliament. Worse, most of these seats are ‘safe’ to varying degrees, meaning they are unlikely to change hands. The outcome of each election depends on a small number of ‘marginals’ where undecided swing voters decide which mainstream party gets to have an MP in that constituency. Whoever wins back the greater number of marginal seats wins overall power (unless of course there’s a tie, in which case either a hung parliament or a coalition government will emerge).
As a result, we see a system where one party will have a clear majority, and will be able to push more or less anything they want through the Commons. To make sure that their MPs all vote the ‘correct’ way, each party will have up to fourteen MPs nominated to be party whips – so-called because they whip their fellow MPs back in line. Failing to follow a party whip’s instruction can lead to an MP being expelled from their parliamentary party, so they will usually toe the line. With the exception of free votes and outright rebellions, MPs will therefore vote the way that their party tells them, rather than doing what their constituents want.