Actually the last *possible* GE - always assuming there isn't some sort of Enabling Act - would be if the monarch passes away when the campaign for a Jan 2025 GE is in progress (this was discussed when QE2 had a health scare during the 2019 election) so February 2025.
If that's your theory, then you could also in theory have perpetual successive monarchs just keep passing away, one every couple of weeks, just before the election, causing the election to be indefinitely postponed until you've worked through the entire line of succession, which would take quite a while.
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