How does first past the post actually work?
First past the post (FPTP) is the voting system used for UK general elections. Each constituency elects one MP, and the candidate with the most votes wins — not a majority, simply more than anyone else.
The crucial detail is the word "most". There is no threshold. A candidate winning 32% of the vote in a four-way contest wins the seat outright, and the 68% who voted otherwise are represented by someone they did not choose. This is entirely normal rather than an anomaly.
The name is also slightly misleading — there is no post to pass, no fixed target. It is more accurately plurality voting.
How government is formed. The party winning most seats — not most votes — normally forms the government. Seats and votes are different things, and they diverge substantially.
The consequences, which are the substance of the debate:
Disproportionality. A party's share of seats can differ enormously from its share of votes. Parties with support concentrated geographically win seats efficiently; parties with support spread thinly can poll millions of votes and win almost nothing.
Safe seats. In many constituencies the outcome is effectively predetermined, meaning most votes there change nothing and campaigning concentrates on a small number of marginals.
Tactical voting. Voters supporting a candidate unlikely to win face pressure to vote for whoever can beat their least-preferred option, which distorts what vote shares mean.
Wasted votes — every vote for a losing candidate, and every vote for the winner beyond what was needed.
The arguments in its favour: simplicity and speed; a clear constituency link between one MP and one area; a tendency to produce single-party majorities and decisive government; and keeping small extreme parties out.
The alternatives include proportional representation systems, the single transferable vote used in Northern Ireland and Scottish local elections, and the additional member system used for the Scottish Parliament and Senedd.
A 2011 referendum on the alternative vote was rejected.