Criminal defense lawyer Philippa Southwell points to the thick ring binders that line her desk in a small office above a bookie's and fast food joint in south London.
More folders are neatly stacked on the floor, in a bookcase and a metal filing cabinet, a hint of her growing caseload.
In recent years, Southwell has specialized in representing a particular kind of client: the mainly young men and boys who are trafficked to Britain from Vietnam to labor in cannabis farms.
Often from poor families, many regard the West as a gateway to prosperity. Others leave weighed down by a duty to provide for parents, brothers and sisters back home.
The Home Office (interior ministry) estimates there were up to 13,000 victims of slavery in Britain in 2013. Victims are most often from Albania, Nigeria, Vietnam and Romania.
Many of the Vietnamese are children when they set off, travelling thousands of miles by foot, boat and lorry over months, sometimes even years, before reaching British shores.
"They will be trafficked usually through Russia, Germany, France. Some of it is done by foot through the woods for days. They sleep in makeshift camps and then they're made to hide in the back of vans in quite squalid conditions," Southwell said.
"They have to remain quiet. They can't move, there is no air. They'll have to urinate in the box they're hiding in."
Once they arrive in Britain, the youngsters are kept as virtual prisoners by their traffickers and forced to tend to cannabis plants
in houses rigged with complex heating systems and high-powered lights to pay off their debt, which can be as much as 30,000 pounds ($46,000).
"It's very dangerous. The electricity's been tapped, there's wires everywhere. The windows are almost always nailed shut so they can't leave," Southwell said.
"There will be filters over the windows so the light can't come in," she told the Thomson Reuters Foundation.
In a recent survey commissioned by the UK Embassy in Hanoi, 70 percent of the returnees from the UK described having worked in cannabis grow house. Photo courtesy of the Association of Chief Police Officers
Although cannabis has been illegal in Britain since 1928, it is the most popular drug in the country.
Up to 2.7 million people consume more than 1,000 tons of cannabis with an estimated street value of 5.9 million pounds every year, according to the Independent Drug Monitoring Unit.
The Association of Chief Police Officers says most of the cannabis consumed in the UK is homegrown, and in the four years to 2011/12, the number of cannabis factories more than doubled to nearly 8,000.
A few minutes' walk away from Southwell's office is Wandsworth prison, an imposing Victorian brick building, where her clients occasionally end up.
But more often, they are dispersed around the country, away from big cities where they can more easily slip under the radar of large police forces equipped with thermal imaging cameras.
When they are discovered, often during a police raid, they are treated as criminals rather than victims, campaigners say.
"To my knowledge, there's never been a prosecution of a Vietnamese trafficking gang for bringing children in for this purpose," said Chloe Setter, head of advocacy, policy and campaigns at ECPAT UK, a charity that works with children who have been trafficked.
"We've actually locked up, prosecuted and convicted more victims unlawfully than we have prosecuted those that are exploiting then," she told the Thomson Reuters Foundation.
Criminal or victim?
In 2013, the Lord Chief Justice of England and Wales ruled that victims of the "vile trade in people" should not be prosecuted when he quashed the convictions of three Vietnamese men, including one of Southwell's clients, for drug offences.
But little has changed since then, experts said.
The police are still arresting minors caught cultivating cannabis while failing to search for clues that might lead to the trafficking ringleaders, ECPAT UK's Setter said.
For example, they rarely investigate numbers in the mobile phones frequently confiscated from the cannabis gardeners.
Duty solicitors called when a minor is detained still advise them to plead guilty to drug-related charges without recognizing they might have been trafficked, Southwell said.
As a result, she is busier than ever, trying to overturn convictions and stop prosecutions.
In 2013, the British government announced a draft bill to tackle rising cases of trafficking and slavery.
The Modern Slavery Bill, expected to be passed before national elections in May, acknowledges that victims of trafficking may be compelled to commit criminal offences.
Though subject to change, the bill in its current form gives trafficking victims who have been forced to commit certain crimes a defence in law. However, campaigners say such victims should not be subject to prosecution in the first place.
Easily groomed, children are a low risk target for traffickers.
Even when they are identified as potential trafficking victims and placed with foster parents or in care, they frequently go missing. One Vietnamese boy who disappeared left a note "apologizing profusely" to his foster carers, Setter said.
"He felt very guilty about leaving but he said, 'They're threatening my sister. I have to go'," she recalled. "That's what modern slavery is. It's not always chains, people locked in. It's a mental form of control."
Once they have served sentences for cannabis-related offences, many of the Vietnamese in Britain illegally face deportation back home.
With a criminal record and limited long-term state support, they often turn to the only network they know, says Mimi Vu from the Pacific Links Foundation, which works with trafficking victims in Vietnam.
According to Vietnam's Ministry of Public Security, 60 percent of all traffickers arrested are former victims, Vu said.
"All they know is the criminal world," Vu told the Thomson Reuters Foundation in a phone interview. "They've been raped, they've been beaten, they've been abused. They've been dehumanized so it's not difficult for them to cross that line."
Back in London, Southwell considers it crucial to get cannabis convictions overturned for her clients.
"It's ... hugely important for them to acknowledge that yes, I am a victim and I'm not a criminal," Southwell said. "I think it's cathartic for them to have that conviction overturned."