One of the most important things to understand about UK general elections is that national vote share does not determine national seat share.
650 results, not one national allocation
The House of Commons is elected in 650 constituencies. Each constituency has one winner under First Past the Post. The national seat totals are simply the number of those local contests won by each party.
The 2024 example
The 2024 result demonstrates the distinction clearly. Labour won 33.7% of the UK vote and 63.2% of seats. The Conservatives won 23.7% of votes and 18.6% of seats. Reform UK won 14.3% of votes and 0.8% of seats, while the Liberal Democrats won 12.2% of votes and 11.1% of seats.
The House of Commons Library described the gap between vote share and seat share in 2024 as the most disproportionate on record.
Geography matters
Two parties can receive similar national vote totals but win very different numbers of constituencies. A party whose support is concentrated in winnable seats may convert votes into MPs more effectively than one whose support is spread more evenly.
Large majorities do not create extra seats
Winning a constituency by 20,000 votes still produces one MP, just as winning it by 20 votes does. Equally, votes for candidates who finish second or lower do not contribute to a national pool used to allocate additional Westminster seats.
Why polling percentages are not seat forecasts
This is also why a national opinion poll showing party vote shares cannot by itself tell you how many MPs each party would win. Turning polling into seat estimates requires additional assumptions or constituency-level modelling.
Sources and further reading
House of Commons Library: 2024 performance of Reform and the Greens ↗ · UKDecides: Polling explained