Six UK parliamentary by-elections have already occurred in 2026, positioning that total as the clear market leader at 69.5% implied probability. The most recent took place on 8 October in Holborn and St Pancras after Keir Starmer resigned his seat, while earlier contests included Gorton and Denton in February plus three simultaneous by-elections on 18 June and the Clacton contest in August. No writs for additional vacancies are currently scheduled before year-end, and the absence of imminent resignations, deaths, or disqualifications among sitting MPs keeps the probability of reaching seven or more relatively low at 23% and 4.8% respectively. Historical patterns of roughly one to two by-elections per quarter support limited further movement in the remaining weeks of the calendar year.