London (CNN) Under cover of darkness, the activists use bolt croppers to cut through the airport's perimeter fence, squeezing through the gap they've created and running across the open tarmac towards their target: a plane, readying for take-off.

In the video provided by the group to CNN, they can be seen dressed in bright pink, unfurling a banner reading "No one is illegal," before securing themselves to one another using pipes and expandable building foam beneath the government-chartered aircraft at Stansted Airport, just outside London.

Their aim: to prevent the plane and its passengers -- 60 people whose immigration status had been ruled illegal, and who were being deported back to Nigeria and Ghana -- from leaving British soil.

It worked, but the protesters, aged 27 to 44, say they had no idea of the trouble they were getting themselves into. Now known as the Stansted 15, they were convicted under charges related to terrorism, and faced potential life sentences.

"Adrenaline was pumping, we were feeling very anxious," May MacKeith, one of those on the tarmac on March 28, 2017, told CNN. "Once we cut through the fence, this amazing sense of clarity, direction and calm took over."

She remembers thinking that whatever the protesters were going through, "this is nothing in comparison to what people who are being snatched out of their beds in dawn raids."

MacKeith pictured in a pink beanie on the tarmac at Stansted Airport.

Airport security personnel were seen on the video arriving within minutes, but the activists remained, lying on their backs, for almost 10 hours as police tried to remove and arrest them. Their actions forced the airport to shut down temporarily.

Speaking in a video posted on Facebook, one of the protesters explained to viewers: "The cabin crew, the pilots have all gone home for the night, and we've just seen all the coaches which were containing detained people have gone back, so we have successfully shut this flight down."

Emma Hughes, another of those who took part in the action, recalls: "At that point, we knew probably that the flight was canceled ... that was a brilliant moment."

Following the Stansted 15's demonstration, activists say 11 people who had been due to be deported that day were subsequently allowed stay in the UK and appeal their cases; they added at least two have been granted leave to remain. When asked about their status, the Home Office said it does not comment on individual cases.

Detainees' emotional testimonies

MacKeith says the Stansted 15 came together after exhausting other options to stop the deportation flights which leave the UK every month. When their requests to members of parliament and lawyers to intervene failed to prevent them, they resolved to take direct action.

May MacKeith is part of the Stansted 15 who prevented a deportation charter flight from taking off.

On their way to the airport, the activists say they took turns reading aloud the emotional testimonies of those who were due to be on board the plane, which had been collected and published by Detained Voices, an independent human rights group which speaks by phone to people being held in detention centers, pending their removal from the country.

The activists said many of those detained fear persecution if they were returned. Their desperate pleas for help had spurred the group on.

Harmondsworth Detention Center near London's Heathrow Airport, where those ruled to be "illegal" immigrants are held prior to being returned to their countries of origin.

In the year leading up to September 2018, the UK government says more than 25,000 people were held in immigration detention centers after entering or attempting to enter the country illegally, overstaying visas, being charged with a criminal conviction, or being refused asylum.

MacKeith, 33, grew up next to one such detention center and spent time as a child visiting those inside. "People I've spoken to who have been in detention say they feel like it's actually worse than prison, because you have no idea how long you're being held for," she says.

'Hostile environment' for immigrants

Under rules introduced by the then Home Secretary (and now Prime Minister) Theresa May, the UK government aims to deter illegal immigration by creating a so-called "hostile environment" -- using ID checks by healthcare providers, landlords and employers to make life so difficult that undocumented immigrants will voluntarily leave, or face being removed by the state.

In a statement, a UK Home Office spokesperson told CNN the government deports those "with no legal right to remain in the UK, including foreign national offenders and failed asylum seekers." It added: "We expect people to leave the country voluntarily but, where they do not, the Home Office will seek to enforce their departure."

Luke de Noronha, an expert in UK deportation at Birkbeck University of London says the situation isn't as black and white as politicians and bureaucrats would like to believe. "People's lives are much more complicated and their connections to the UK are much richer than the Home Office give them credit for."

De Noronha says he has met young men in detention who have lived in the UK since they were children. "On the outside of a detention center they would appear to be a kind of Londoner, or Mancunian ... These are people who have British families, British children, British memories and British accents."

The UK Home Office told CNN that while there is no fixed term on immigration detentions "it is a myth that we detain people indefinitely," adding "the law does not allow it."

In statistics published on its website for the full 2017 calendar year, two-thirds of those detained were held for fewer than 29 days, while the overwhelming majority (95%) were held for less than six months. A small percentage -- 1,160 people, or 4% -- of those held were detained for a longer period.

While de Noronha admits some detainees do have criminal records, he says the UK Government shouldn't use that alone as a cause for deportation. "Should someone selling weed when they're 19 be enough to justify their deportation, even if they moved here aged two?"

Secretive charter flights

The UK does not publish data on its deportation charter flights, but following a Freedom of Information request in 2018 , the Home Office said 40 planes left the country in 2017, heading for countries including Pakistan, Nigeria, Ghana, Jamaica and Albania. In the same year, more than 4,000 people were issued with deportation notices, but 2,600 returnees were withdrawn from flights following last-minute legal action or for other reasons, the FOI request revealed.

Secretive, and from undisclosed locations, de Noronha calls the charter flights "insidious" because no independent witnesses are are present when deportees embark, or on board the planes to observe what goes on.

CNN reached out to Titan Airways, the charter airline whose plane the Stansted 15 blocked from taking off but did not receive a response.

Following their arrest, the Stansted 15 were initially charged with aggravated trespass, but the case against them was later upgraded, and they were eventually found guilty of endangering safety at an aerodrome, under the 1990 Aviation and Maritime Security Act -- a law brought in after the Lockerbie bombing.

It carries a potential sentence of life imprisonment.

The Stansted 15 activists were found guilty of endangering safety at an aerodrome.

Judith Reed from the Crown Prosecution Service said in a statement when the group were convicted that the activists had "placed themselves, the flight crew, airport personnel and police at serious risk of injury or even death due to their actions on the airfield."

Amnesty International described the conviction as a "crushing blow for human rights" while the activists' lawyer Raj Chada from Hodge, Jones & Allen told CNN it was an abuse of power and of a charge that should only be used for violent acts of terrorism.

"Never in a million years did they think they would be classified as terrorists by the state and be put on trial for such a charge," he said, adding that the convictions would affect their future careers, and limit their ability to travel.

'The only people in danger were the deportees'

Charity worker Hughes gave birth to her son 10 days after the guilty verdict was announced. Prior to the sentencing on Wednesday, she told CNN she feared the prospect of being separated from her baby.

The trial "was incredibly stressful and at a time when I wanted to be thinking about upcoming motherhood and my baby, instead I was really having to worry about being separated from my child if I'm sent to jail," Hughes said.

Emma Hughes risks being separated from her newborn son, should she receive a jail sentence on Wednesday.

She added that the charges against the group "matches nothing that we did that night ... We didn't endanger anyone, the only people in danger were the people due to be deported on that plane."

Hughes says she now feels even more connected to those being removed on deportation charter flights. "It really opened my eyes to how the deportation system is separating families and ripping them apart."

This week, three of the activists were given suspended jail sentences, while the remaining 12 -- including Hughes -- were handed community orders.

But despite the conviction, and having to juggle community service while caring for a newborn, Hughes says she can't regret stopping the plane.

"We know that there were two victims of trafficking who should have never been on that flight ... These are people who never should have been deported, so I'm still glad we stopped that." The Home Office said it does not comment on individual cases.

Defendants and supporters gathered outside Chelmsford Crown Court on the first day of the trial in 2018.

MacKeith says she believes the actions of the Stansted 15 have prompted a change in attitudes, with more people arguing against deportations and detention centers -- a fight she feels "deeply privileged" to be part of. "I really hope that leads to some significant and lasting change."