Sometimes it takes the older generation of members and leadership to die out for these beliefs to shift.

https://en.wikipedia.org/wiki/Planck%27s_principle

"Science progresses one funeral at a time".

Yeah, this might well not be any individual human being changing their mind, but rather new human beings less inclined to be anti-nuclear-power now forming a critical mass within the UK Green Party.

Good point, good chance very few individual minds actually changed their stance.

It's hard to convince people to change a stance they made a part of their political and personal identity.