You need to be careful with the methodology: voters, parties and campaigns behave differently depending on the electoral system in place, so re-running FPTP vote totals through a different seat-allocation formula doesn’t necessarily reproduce what would actually have happened.
Under PR, more parties would likely stand candidates, campaigns would shift, and voters freed from the incentive to vote tactically (about a third of 2024 voters said they had) might have voted differently in the first place. This is an informed estimate, not a genuine alternative history.
The source: these projections come from the Electoral Reform Society (ERS), a campaigning organisation for electoral reform, so its figures are worth reading with that advocacy position in mind.
Its methodology goes beyond simply reallocating raw FPTP votes: it commissioned a YouGov poll asking voters how they would have ranked parties in order of preference, and used those preference patterns, by region, to model transfers and list allocations. Northern Ireland’s own party results weren’t separately modelled, so its 18 seats are held constant throughout.
Source: A System Out of Step: The 2024 General Election (ERS, December 2024) — https://electoral-reform.org.uk/latest-news-and-research/publications/a-system-out-of-step-the-2024-general-election/
| Party | Actual (FPTP) | STV | AMS (50:50) | AMS (68:32) | List PR |
| Labour | 411 | 227 | 228 | 282 | 194 |
| Conservative | 121 | 148 | 139 | 134 | 147 |
| Liberal Democrats | 72 | 71 | 73 | 58 | 78 |
| Reform UK | 5 | 90 | 100 | 75 | 109 |
| Green Party | 4 | 67 | 71 | 57 | 83 |
| SNP | 9 | 16 | 16 | 18 | 16 |
| Plaid Cymru | 4 | 8 | 4 | 5 | 5 |
| Independents | 5 | 5 | 0 | 2 | 0 |
| Northern Ireland parties* | 18 | 18 | 18 | 18 | 18 |
| The Speaker** | 1 | 0 | 1 | 1 | 0 |
*Held constant at the actual total. **Modelled only under AMS, given the uncontested nature of the seat.
Labour’s majority disappears under every variant. Even AMS (68:32), the least proportional model tested, gives Labour only 282 seats, short of the 326 needed. Under STV it falls to 227. Labour stays comfortably the largest party throughout, but would need at least one coalition partner, and in most scenarios two.
Reform UK’s fortunes are transformed, rising from 5 seats to somewhere between 75 and 109, roughly proportionate to its 14.3% vote share. The Green Party sees the same pattern less dramatically, rising from 4 to between 57 and 83.
A genuinely multi-party Commons emerges in every scenario, with the ERS suggesting the likeliest outcome across all three systems would be a Labour-led coalition with the Liberal Democrats and/or Greens, against a right split between the Conservatives and Reform UK.
The specific system chosen still matters. Moving from AMS (50:50) to AMS (68:32) alone shifts around 54 seats towards Labour — a reminder that “PR” is a spectrum, not one outcome.