The Greens have begun their annual conference by setting a target to become England’s third-biggest party while promising to maintain direct protests against policies such as fracking and support others who do.

A month after taking over as co-leaders, Sian Berry and Jonathan Bartley unveiled what they called a transformative programme, including measures to redirect economic policy away from growth towards general wellbeing.

As well as a proposal to replace GDP as a yardstick of economic health, with a gauge of how much leisure time people are able to enjoy, announced before the speech, Bartley suggested the Department for Work and Pensions should be renamed the Office for Welfare and Wellbeing, with the Home Office becoming the Department for Security and Welcome.

Berry, a London assembly member, began by saying: “To the Lib Dems: we are ready to take your place as England’s third party.”

The Greens are fourth by number of councillors, but have only one MP, Caroline Lucas, and still trail behind Ukip as well as the Lib Dems in the opinion polls.

In a rapturously received speech in which Berry and Bartley delivered alternate passages, Bartley, who is leader of the opposition on Lambeth council in south London, praised the actions of direct campaigners opposing fracking and enforced immigration removals, warning that police and court action against such people was hugely disproportionate.

Bartley hailed three activists jailed for up to 16 months for opposing fracking in Lancashire as being “on the right side of history”. The trio announced on Friday they will appeal amid growing anger about the severity of the sentences.

He also called on prosecutors to drop terrorism charges against 15 activists who locked themselves together around an immigration removal charter flight to prevent its departure from Stansted.

“The Stansted 15 acted as our conscience,” he said. “The conscience of every person in the country who believes that Britain is better than to shackle people and deport them into danger.”

He condemned “the hypocrisy of those who celebrate the suffragettes and the anti-apartheid activists in one breath but then turn their backs on those who fight for human rights today”.

In a section on Green policies intended to respond to expected changes to employment amid rapid technological advances, Berry said employees should be guaranteed the right to paid training.

“All workers should be able to grow and learn throughout their careers, get trained in the things they need to know for their next job, improve how they do the one they have, or learn a new skill or trade,” she said.

One policy idea is to formalise a four-day working week, which has been endorsed by the TUC.

Speaking to the Guardian after the speech, Berry said the proposal, as well as one for a universal basic income, was being adopted by others, adding: “As the world changes, our policies become the right solutions.”

Bartley said: “The big picture is that we cannot go on as we are. We have to get off this consumption-drive, GDP-worshipping treadmill, which is bad for people and bad for the planet.”

The pair said there was no contradiction between them seeking a mainstream role for the Greens and continued support for direct action.

The Stansted protesters seemed to be targeted as a warning to others, Bartley said: “The fact they’re invoking terror charges – this is a way of discouraging more action like it.”

Berry said: “That kind of radicalism is ready to be in the mainstream – it should be in the mainstream. People are in the mood, nationally, to look for more radical options.”