Keir Starmer’s September 1 resignation as MP for the safe Labour seat of Holborn and St Pancras has triggered an unscheduled by-election whose timing remains unclear pending the writ. With Labour’s candidate selection process only just opening and no nominees yet confirmed, trader pricing on Polymarket fragments across dozens of potential Labour figures, Green leader Zack Polanski, independents, and others. Local results showing Green strength in the 2026 Camden elections and the 18.9% independent share from 2024 keep multiple outcomes competitive. The absence of a confirmed field, the constituency’s progressive profile, and the risk of vote splitting among challengers sustain tight implied probabilities until nominations close and clearer signals emerge on frontrunners and turnout dynamics.