They're militant left. But I don't mean Militant Tendency!
Greens exist in a strange vacuum where they don't have any policies actually pinned on them for any length of time. What's the average Green voter's actual understanding of the party they vote for? They're the Left protest party who also have some historical relevance to environment politics (which has been undermined by both Labour and the Lib Dems making the environment part of their core pillars) But to describe what I mean:
They're to the hard left of Labour. One policy they proposed at the last election was to not just scrap tuition fees (which I am a fan of) but to repay everybody who had to pay them in the first place. They're also for policies like universal basic income - which, again, I am for in principle. But it's unproven and a major challenge to both implement and maintain in a free economy. The Greens put policies like that front and centre, because that's who they are - a militant left protest force who just has to be credible enough for people to throw votes at them instead of Labour and the Lib Dems when either of those parties do dumb things.
Basically, they're ideological lefties who live perennially in protest - a description well-suiting Jeremy Corbyn.
Obviously this is my own opinion and I'm not a Green - for all I know the party is actually incredibly moderate and just naiive. But it really doesn't surprise me to hear about paid-up Green members trying to pull Labour out left.
EDIT:
I don't think I'm enough on a subject matter expert on the Green Party to go further than the above to try and explain what I am getting at, so to switch to a different topic:
Are Umunna and Hunt actually trying to create some Gang of Four style split in Labour, like John Prescott accused, or are they really just going to be doing what they say on the tin - be a forum for centrist discussion in the Labour Party?
I don't really know much about either Umunna or Hunt to comment. What are their politics like? Is it different enough from normal Labour to make them a credible fulcrum for a split? Who'd go with them? Kendall?
I'm thinking back to what I've heard about the origins of the SDP over the years. I've heard about the characters involved - how Shirley Williams was a Liberal in the wrong party for years, for a start. I'm wondering if an actual new party could arise from a group founded by those two, and if it'd be any more credible than just 'hey guys, remember Blair?'