Labour have a big majority in the HoC, Keir Starmer has no fear of being de throned any time soon.
If the next GE is 2029, then Keir Starmer has no fear of a challenge until spring 2028 and that would be dependent largely on the economy.
I can’t see an obvious contender. The obvious and most ambitious person is prob Wes Streeting, but I don’t see him being very popular really