London (CNN) Members of a human trafficking gang were jailed last week in the UK, described by the trial judge as the biggest modern-day slavery network ever identified in the country.

The gang is thought to have made more than £2 million ($2.5 million) from trafficking its victims from Poland.

Five men and three women, all originally from Poland, were sentenced for a total of more than 55 years. Some had been sentenced in February, but reporting restrictions on the case prevented them being named until last Friday.

Police believe there were up to 400 victims in all. Aged from 17 to their 60s, they were promised jobs and accommodation in the UK, but were forced to work at farms and waste recycling centers in the West Midlands, and had most of their salaries taken by traffickers. Some worked for just 50p (63 cents) an hour.

They were made to live in cramped housing, some without working toilets or heating. Some victims said they were forced to wash in canal water and fed out-of-date food; some were beaten.

A property where some of the victims lived.

Treated as 'commodities'

Police began investigating in February 2015, after two victims escaped their captors and were helped by slavery charity Hope for Justice.

Detective chief inspector Nick Dale, senior investigating officer, said the gang treated its victims as "commodities purely for their own greed."

He said that if victims objected to their treatment, they were beaten or threatened with violence and were told family members in Poland would be attacked. Some were told they would be taken to the woods to dig their own graves, Dale said.

"Most felt powerless to escape, with no knowledge of the area, little or no English language skills, and no-one to turn to for help," Dale added. "Their lives were reduced to misery and they all have the physical and psychological scars of their exploitation."

'A growing problem'

The charity Salvation Army provides outreach and accommodation for traffickineg survivors, and supported some of the victims in this case. Emilie Martin, of the group's anti-trafficking and modern slavery unit, told CNN that labor trafficking is a growing problem in the UK.

"It used to be that our referrals were predominantly for female exploitation and forced prostitution, but with awareness training people are starting to notice different kinds of exploitation and police are realizing labor exploitation is very much on the increase," she said.

She added that exploitation of Eastern Europeans was increasing.

In 2017, 6,837 potential slavery victims were identified in the UK, according to the 2018 UK Annual Report on Modern Slavery. Albania was the most common country of origin among adults, followed by Vietnam and China. Romania and Poland were also among the 10 most common origin countries.

Labor exploitation was the most reported, followed by sexual exploitation.

In May 2018, a report by a group of UK lawmakers said that sex trafficking was taking place on what one lawmaker called an "industrial scale" across England and Wales, with huge numbers of women, predominantly from Eastern Europe, being trafficked into "pop-up" brothels. More than a third of potential victims were Romanian.

It also noted that organized crime groups "increasingly dominate the sexual exploitation of women."

In 2015 the UK introduced the Modern Slavery Act, which gave crime agencies new tools for tackling trafficking, including a maximum life sentence for perpetrators. In an independent review published earlier this year, a lawmaker described the act as "world leading," but said there were still too few convictions.

Martin said labor trafficking victims weren't always vulnerable people, but simply job seekers who were being tricked by deceitful traffickers into going to the UK, and then exploited.

"Some people are very educated, well qualified individuals applying for what looks like a genuine role only to find themselves in a situation of exploitation," she said.