I'm from the UK and don't anticipate any political impetus for leaving the WHO even if the Tories return to power. I feel like the UK discourse did not blame the WHO for COVID scandals as much as the US's did. Googling around I did find this petition during Sunak's term, but it ranked low in terms of signatures for petitions during that Parliament: https://petition.parliament.uk/archived/petitions/648609 .
I still think for the most part the WHO's actions during COVID were all made in good faith, even if mistakes were made.
The WHO being outside politics is good, it means it can focus on health/science and not worry about appealing to electorates.
I think Trump has a point though regarding China's contributions, China isn't a poor country but the WHO doesn't really have any way of forcing it to pay more.
Starmer is many things wrong but one thing he won't do is renegade on international treaties and agreements.