On a mild Tuesday night on 28 March last year a minibus containing 15 protesters inched up to the perimeter of Stansted airport away from the public terminal and towards the cargo field. Shaking with adrenaline, they clutched at boltcutters to rip a hole through the barbed-wire fence and scattered across the tarmac. Within eight minutes, their lives were changed for ever.

Last week, 623 days later, the Stansted 15, who that night had locked themselves on to a Titan Airways Boeing 767 secretly chartered by the Home Office, were convicted of terrorism offences at Chelmsford crown court. The group of nine women and six men had been protesting to stop the deportation of 60 vulnerable migrants to Ghana, Nigeria and Sierra Leone. The police initially charged the group with aggravated trespassing, an offence carrying a maximum three-month custodial sentence.

But the attorney general, Jeremy Wright, granted permission to prosecute the group under the 1990 Aviation and Maritime Security Act, a law passed in response to the Lockerbie bombing in 1988. In February they will face sentencing for endangering the safety of the airport, an offence that carries a maximum of life imprisonment.

“I went numb,” says May MacKeith, 33, who spent 10 weeks in the dock with her co-defendants. “I feel heartbroken that the jury – knowing all the information they did, and everything we told them about why we did it, and the facts of what happened – couldn’t find us not guilty.”

“It’s been a huge endurance test,” says Jyotsna Ram. “There is a toll on all our physical and mental health. There was no personal gain for us.”

Eleven of the 60 passengers who were to be deported on the blocked aircraft now legally live in the UK, which the Stansted 15 count as a triumph. “Half of the Home Office’s immigration decisions are overturned on appeal,” says MacKeith, “but the government are denying people that right – you can’t appeal if you’re deported first.”

“We didn’t put anyone’s life in danger,” says Ram. “We were there for 10 hours. There was nothing to suggest we endangered anything or anyone. The police and staff were joking with us, laughing with us, they took chewing gum out of one of my co-defendant’s mouth because it got stuck in her hair – they were helping. It was very friendly on the night and no one who was present told us we were putting people in danger or what we were doing was dangerous.”

Last week’s verdict was described by Amnesty International last week as a “crushing blow for human rights in the UK” and attention has focused on the judge, Christopher Morgan, who told the jury to disregard all evidence put forward by the defendants that they had acted to stop human rights abuses. Instead, he instructed jurors to consider only whether there had been a “real and material” risk to the airport.

“I don’t think [the jury] felt they had a choice when an authority figure presented it in the way they did – the judge introduced the phrase ‘inherently dangerous’ in relation to what we did,” says MacKeith. But it was the attorney general’s decision, adds Ram, that dealt a major unprecedented blow.

“We have asked through the Crown Prosecution Service, the courts, our MPs and Amnesty International, who have written to ask what the attorney general considered when it came to the facts of our case and why the charges were upped,” she says. Neither woman admits to any regrets. The 15 were hosted by local families during the trial and supported by the Quakers, without whom, they say, it “would [have been] impossible” to even attend court every day. Relationships have been sorely tested and parents and loved ones have suffered. One member of the group is eight months pregnant; others have lost work.

MacKeith is sad about the impact on her mother, too, who was in court almost every day. “It’s taken the wind out of her sails and I feel really bad for involving her.”

MacKeith has spent most her career working in climate change organisations. She grew up near Oxford and throughout her childhood went to weekly demonstrations at Campsfield House – an immigration detention prison – with her father, an academic editor.

The protesters stopped a plane chartered by the Home Office from taking off. Photograph: Kristian Buus/In Pictures via Getty Images

Ram, also 33, arrived in the UK from India via a US boarding school as an undergraduate at Oxford Brookes university. She went on to study at the Bartlett School of Architecture at University College London, and has a PhD in sustainable design and urban planning. “I still get anxiety about my citizenship process,” she says quietly. “It took a long time and it was difficult. Some of my white friends who also weren’t born here had a much easier time.”

MacKeith describes the “hostile environment” first outlined by Theresa May as home secretary in 2012 as deliberately divisive and overtly racist. “It’s been a slippery slope for a really long time,” agrees Ram. “Charter flights [for deportations] were put in under New Labour but now it feels that anyone who is black or brown is game.” MacKeith and Ram met through friends of friends a couple of years ago. Weekly Thursday meet-ups were fixed between members of the group, who are aged between 27 and 44 and live in London and Brighton. Discussion was focused on social issues and how to tackle them, but the pair balk at being described as professional activists.

“What is a professional activist?” asks MacKeith. “If we were professionals, we would be getting paid. I despise that phrase, because it’s used in a derisory way.”

Ram agrees: “I’ve never done anything like this action before. For me, I’ve found a caring community and friends within a group who are really engaged in their communities and with different struggles.” Part of what Ram feels has “made the action strong and given it reach” is that “it is not white and middle class, it is so diverse in backgrounds, in sexuality, gender and race.”On Tuesday evening last week, a demonstration to protest against the Stansted 15’s guilty verdict was held outside the Home Office. More than 1,300 attendees turned out to hear MPs Diane Abbott and Clive Lewis speak alongside Lesbian and Gays Support the Migrants.

MacKeith and Ram, meanwhile, remain optimistic that their convictions will be overturned on appeal next year. And although they are defiant, they are keen to avoid being depicted as martyrs for the cause.

“There are people who are indefinitely in detention,” says MacKeith. “We’re not in prison. They don’t know even when they’re going to be released.”