The Greens’ new leadership duo of Jonathan Bartley and Siân Berry have promised the party will make a stand against “vapid centrism” in UK politics, making it clear that they consider Jeremy Corbyn’s Labour part of this tame establishment.

Bartley and Berry were strong favourites throughout the campaign to choose the party’s leadership in England and Wales, and won almost 75% of the votes cast. They take over from the previous pairing of Bartley and Caroline Lucas.

The Greens have been badly squeezed in recent years, seeing their vote share more than halve between the 2015 and 2017 elections, something put down in part to Corbyn’s approach with Labour.

But the new leaders told the Guardian they wanted to counter the idea that Corbyn had moved into their territory, with Berry highlighting what she said was Labour’s equivocation over areas such as Brexit and migration. “It’s not tame – it’s a betrayal. They’ve utterly fudged those issues,” she said.

Bartley said: “There’s handwringing over Brexit. They’ve fudged Heathrow and airport expansion, Hinckley and nuclear power. They aren’t committed to proportional representation. The Labour manifesto had no real wealth redistribution in there. There was income redistribution, but they’re not tackling the elephant in the room of wealth distribution.

“There’s still this commitment to credit-fuelled growth across the every sector of the economy. There’s subsidies for fossil fuels, subsidies for the commercial arms trade. This is a social democratic party. It’s not the radical alternative the country needs.”

In May, Lucas said she would not stand again with the aim of “making space for other people”, while pledging to remain a Green MP.

Bartley, leader of the Green opposition on Lambeth council in south London, then joined forces with Berry, also a London councillor, in Camden, and a member of the London assembly.

The other two candidates were both standing alone: Shahrar Ali, an academic and former deputy leader; and Leslie Rowe, who stood for parliament for the Greens in Richmond, Yorkshire, in 2005, 2010 and 2015.

The party elects its full leadership team every two years. Lucas, the Greens’ sole MP and best-known figure, spent two years as joint leader with Bartley.

Berry and Bartley say they hope to raise the profile of Green politicians across the country, such as the new Green lord mayors of Sheffield and Bristol, Magid Magid and Cleo Lake, and councillors newly elected in May’s local polls.

Berry said: “In the last couple of years we’ve really demonstrated the role we have in being that voice for communities, of opposition.”

The parallel vote for deputy leader saw the incumbent, Amelia Womack, voted in again.

Among Womack’s opponents was Aimee Challenor, a 20-year-old activist from Coventry who served as the Greens’ equality spokeswoman, and was seeking to become the first trans person to take the role.

However, Challenor quit the race and has since left the party following the conviction last month of her father, David – who was also her election agent – for torturing and raping a child at the family home.

One of the most pressing tasks facing Bartley and Berry is to oversee an independent inquiry into safeguarding lessons from the case, notably why the party took no action against David Challenor for 18 months after he was charged with the offences, for which he was jailed for 22 years.

His first court appearance over the allegations took place in November 2016. But before the trial, which began in August this year, he acted twice as his daughter’s election agent, at the 2017 general election and in May’s local polls.

During this period he was allowed to remain active in the party, even though the Coventry Pride charity, for which he had volunteered, barred him after the first court appearance.

As well as the inquiry into the case, due to report in November, the new leaders are awaiting the result of a previously-established review into the Greens’ organisational structures.

Bartley said: “Our platform was very clear when we stood – that we want to professionalise the party.” Bartley and Berry must also seek to reinvigorate it.