Edit: This is a bit of a zombie market, born by my mistake. See edit history if you're interested (but it's not very interesting).
This market resolves YES if the number of seats contested at the next UK general election is less than or equal to 620. Otherwise NO.
(The number of seats is 650 and will continue to be so. There have been discussions about reducing the number, but this is not recommended by the most recent boundary review.)
https://en.wikipedia.org/wiki/2023_Periodic_Review_of_Westminster_constituencies
I will bet in this market.
Turns out I can't read. The review recommended keeping 650 seats, unlike previous reviews. They're redrawing boundaries but not changing the number of seats.
https://en.wikipedia.org/wiki/2023_Periodic_Review_of_Westminster_constituencies