That said, is it for us in the UK to say that a one-state solution is the correct one? I think that's overstepping our boundaries.
We're already overstepping our boundaries by arming Israel. That's a much worse case of solutionizing from afar. It's a rather final solution from afar.
This stance is one that says "This is morally wrong". Supporting a one state solution is just saying "We think this is a good road to peace and will support all efforts to go this way". It's not a demand on anyone. I'm sure green party members will be happy to see anything that stops the killing.
I don't really see what you're wringing your hands over.