Just before 10am on a rainy Thursday in southeast London, a small group of men gather in a pokey corridor inside Bexley magistrates court to await their punishment. They don't chat much and they have little in common – one is in his 70s, another is a self-employed T-shirt seller, while a third is just out of prison – but they share the status of convicted domestic abusers.

As they trail one by one into court for a morning of sentencing, common themes emerge: alcohol, anger and then remorse and sometimes a promise that it will never happen again. But a major change in the working of the criminal justice system only becomes obvious when Karl Langridge, 41, takes his turn.

He accepts that he had too much to drink in the pub before heading to the home of his business partner and brief short-term partner. He refused to leave the house when asked, then hurt her wrist when he wrenched a phone from her. So she called the police.

What happened next was captured on a camera pinned to the breast of an officer that answered the call. Prosecutor Clare Carey presses the button on her laptop to transmit wirelessly the footage to a widescreen television screen in the court room.

The magistrates see the flustered victim open the door and beckon the officers in. After a rudimentary search of the substantial property, officers found Langridge hiding behind the kitchen door.

What then happens is hard to ascertain from the footage, but there is a scuffle as officers try to arrest him. “Behave,” says one officer, just before the clip ends. “You're on camera, sir.”

Criminal record: an officer wears a new Reveal camera

The footage was cited by police and prosecutors as key to why Langridge pleaded guilty, which he denies. “What I pleaded guilty to, I was guilty of,” he said after the hearing in which he was ordered to complete 120 hours of unpaid work. “It had nothing to do with the body camera.”

But what happened in Bexley is likely to become the norm. In 2016, the Metropolitan Police will introduce the world's biggest single roll-out of 22,000 body-worn cameras. By the end of the year, the majority of front-line police officers across the country will have access to one.

Published trials by Hampshire police on the Isle of Wight, Essex police, and Scotland Yard, suggest that the cameras are particularly effective for capturing evidence in domestic abuse cases.

In one case, a camera captured evidence of a man who punched his partner in the face and hit her with a frying pan in front of her children. The woman gave a statement, but later withdrew it. The evidence from the video camera was enough to charge the man. In another, the camera captured the “threatening demeanour” of a suspect and the emotional anguish of his parents whose home had been smashed up after they refused to give him money to buy heroin. The suspect was charged and remanded in custody.

But it is not the only perceived potential benefit. The largest pilot study by the Metropolitan police – which led to its £3.5m investment in the cameras – set out to show that malicious complaints against police would fall, oppressive behaviour by police – particularly on stops and search – would decline, and evidence gathering would improve. The findings of the report were mostly inconclusive, though malicious complaints against police were down. “Where officers were wearing cameras, there was a 33 per cent reduction in allegations against them,” said Nerys Thomas, head of research at the College of Policing.

It was enough for the Mayor, Boris Johnson, to claim that “our trial simply scratched the surface and, once rolled out, these cameras have massive potential to help our officers continue their great work in fighting crime and keeping our city safe”.

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The market for the cameras is dominated by two companies, Britain's Reveal and the US-based Taser International. While Reveal supplies more forces, Taser bagged the world's biggest single force contract with the Metropolitan Police and followed it up with another large deal with Greater Manchester Police.

I was given a demonstration of the technology at Reveal's marketing HQ on board a boat moored on the River Thames in west London. The company has experienced rapid growth across the world on the back of the expansion of body-worn cameras in the criminal justice system. Its turnover last year was £6m. It is now selling into 30 different countries and has won tenders for the police in Hong Kong and Singapore, and secured contracts with the French Ministry of the Interior, says chief executive Alasdair Field.

“The UK has the highest density of cameras per officer,” says Field. “Almost every officer in the UK within the next year will have access to a camera for their front-line duty. I would say the UK is two years ahead of the US, but they're catching very quickly.”

The industry's growth has not always been driven by positive reasons. President Obama demanded millions from Congress for the technology in the wake of riots that followed the shooting of black teenager Michael Brown by a white officer in Ferguson, Missouri.

In Britain, the scandal of mistreatment of youngsters at G4S detention centre in Medway, Kent, prompted prison inspectors to demand cameras for all warders working in youth detention centres. The months-long saga and multimillion pound cost of the Plebgate affair could have been resolved in minutes if one of the officers had been wearing a body-worn camera. “The original use of body-worn cameras was to gain better evidence,” says Field. “Nowadays, because of the political agenda and Plebgate, it's a bit more about keeping an eye on the officers. It's a real shame that it seems to have shifted from evidence-gathering to making sure the police are doing the right thing.”

The camera revolution started with a small-scale Home Office project in Plymouth eight years ago using 50 bulky camera kits. The camera unit was mounted on a headband and linked by wire to a hard drive. Officers chosen to test them reported side-effects including “nausea after prolonged use”. Since then, the cameras have developed into small self-contained palm-size units that clip on to police uniforms. The Reveal camera has an outward-facing screen, so it's clear to the suspect or witness that they are being filmed. It's simple to operate, with the flick of a switch. Once filmed nothing can be deleted from the unit and the information is fully encrypted. “If this gets lost, stolen or forgotten on the kebab shop counter, no-one else will be able to replay that data,” says Field.

After recording, the unit is dropped into a multi-dock port, where the pictures are uploaded to a secure server within police buildings. An audit trail is created that shows who has used it, and when, not only useful for processing the evidence through the criminal justice system but to guard against leaks. About a fifth of the 840 police corruption cases passed to the police watchdog for investigation over three years involved the misuse or passing on of police information, according to a 2012 report by the Independent Police Complaints Commission. “If a celebrity was arrested and filmed, I could tell you every officer who has logged on and viewed that footage. That protects the privacy of that celebrity,” says Supt Adrian Hutchinson, the lead for mobile technology at the Metropolitan Police, which is using Taser International's Axon system.

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The leak of sensitive information is a key concern of privacy campaigners. While data on the systems of the market leaders are encrypted, cameras being used by other forces still are not. “The best standards should be high levels of encryption so that if they're lost, the pictures couldn't be flooded on to YouTube,” says Tony Porter, the surveillance commissioner and a former police officer. “At the moment there's not 100 per cent adoption of encrypted cameras. Those forces that don't yet have encrypted cameras have spoken about having very firm policies about merging and uploading as soon as possible after an event so that risk is negated. The question I ask: is that sufficient? My answer would be that it's not. All body-worn cameras used by law enforcement should be encrypted.”

The College of Policing sets out the rules on retaining, using and disposing of police information. The policy at the Met is that if an officer is going to make a note in their pad, then they should start filming. If the footage is not to be used for a criminal investigation – or subject to a complaint against police – then systems automatically delete the footage after 31 days. The College says that “extremely strong justification” would be needed to keep footage that did not contain evidence for any longer than that.

Difficult decisions remain to be made over when footage would be publicly released, for example to trace suspects. Superintendent Hutchinson says the US experience – which has a much more liberal attitude to the release of police recording information – has seen huge amounts made available for potential public viewing. In one case, footage showing a child having its throat cut that was shown on television prompted a huge public backlash, according to the officer. “We will not be routinely releasing footage unless it's absolutely appropriate,” says Supt Hutchinson. “What I don't want to see is a 15-year-old who makes a mistake which 10 years later comes back to damage their life chances. That's not fair.”

Senior officers at the Metropolitan Police say that they would consider showing footage to a select group of community leaders to prevent unrest if there were erroneous rumours of police wrong-doing. But if there was police wrong-doing, the guidance suggests that it might only be released if an officer was convicted of a criminal offence.

“Forces will need to consider how transparent they want to be, and this will be critical in preparation for high-profile incidents where BWV is available,” says the Met police report, adding that “routinely sharing footage may not be desirable”.

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Britain's Chief Inspector of Policing, Tom Winsor, has been scathing about the police's use of technology. In his maiden speech on taking the job in 2013 he highlighted how some 2,000 different IT systems are in use in 43 different police forces. While forces buy their own gear, they do not always operate within their own force borders. When a major incident takes place, officers are drafted in from other forces, bringing their own equipment. The big two in the body-worn camera market – Reveal and Taser – are working on technology that will allow recordings from one system to be used by their rival's software, but it's not in place yet.

The criminal justice system has a patchy record in keeping information safe. The Crown Prosecution Service – which will be handling the data obtained from the cameras for court cases – was fined in November after burglars stole unencrypted laptops containing videos of sex abuse victims detailing how they had been groomed and attacked. The Information Commissioner's Office said that the CPS had been “complacent” in protecting that information.

And despite the pledge of Justice Secretary Michael Gove for a technological revolution, and the advent of the first “digital courts” such as Bexley, the security of the data becomes problematic once it leaves police servers stored in protected headquarters. The CPS is not yet linked up to the system and Field, of Reveal, says that a DVD player used in court would not be able to read encrypted footage.

“Generally speaking in the UK, not many constabularies are connected to the outside world, so the majority of them are resorting to burning DVDs… it's not a restriction of our software but a restriction of the infrastructure of the criminal justice system. It's odd that when you have a connected world where the internet is ubiquitous… that we're still in an age when it's difficult to get it from one part of the justice system to another.”

The Met says that it was committed to ending the use of the computer disk, and will send any footage used for court cases on a protected email link to prosecutors. “We want to bring about the end of the disk. A disk could get lost, it corrupts, or it gets damaged,” says Supt Hutchinson.

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Four years after the killing of gangster Mark Duggan sparked nationwide riots, senior Scotland Yard officers returned to Tottenham in December to explain another fatal shooting by police.

Jermaine Baker, 28, was shot dead in a police operation before an apparent attempt to spring two gangsters from a prison van. The police – conscious of the misinformation spread after Duggan's shooting – wanted to address “any concerns raised” by the community.

Top of the list of those concerns was the use of personal cameras by officers involved in Baker's shooting. The coroner at the inquest of Duggan concluded that if three officers had cameras on that operation, they could have avoided the “difficulties and distrust” that led to rioting in the capital.

But at the meeting in December – despite the coroner's recommendations – senior officers and the police watchdog told the 150 people assembled that there was no CCTV, and no footage from body-worn cameras to show what happened. The officers were repeatedly challenged – why not? “When I walk the streets of London, there are thousands of CCTV cameras that pick me up when I don't want them to – it's an everyday reality for us,” says Stafford Scott, a campaigner who helped organise the meeting. “But these are situations when we absolutely need them to win the trust and confidence of the whole of society.”

After Jermaine Baker's shooting, Sir Bernard Hogan-Howe, head of the Metropolitan police, said that his force was struggling to find kit that could be used for armed covert operations. “What I'm not going to do is equip surveillance officers with kit which allows them to be identified, particularly if you've got firearms officers who I assume are going to be facing people with guns,” he told journalists.

But the equipment is available and being used by police forces, says an industry source. One company sells cameras that are fitted in buttons. “The equipment does exist, the solutions are available,” says the source who declined to be named because of the nature of his work.