A drop in small boats illegally reaching the UK is forcing people smuggling gangs to adopt new tactics, a BBC investigation has found.

And there are claims of more "mega-dinghies" being prepared, of the type seen arriving on UK shores in recent weeks.

The image of a 15-metre long small boat carrying a record 230 migrants towards the UK drew condemnation from both newspapers and politicians.

Ministers were accused of "losing control" of illegal Channel crossings, and the French police again found themselves criticised for not intervening.

But evocative as the picture was, it also represented a new strategy by the increasingly frustrated people smugglers we encountered on France's northern shores - who attacked our crew with stones, and injured our cameraman.

They are running low on small boats, we have found. And once rival criminal groups are now having to work together by loading more migrants on fewer, larger vessels - just like the "mega dinghy".

The UK and France are announcing measures to increase the number of police officers on the French coast in order to try to stop the migrants.

The latest initiative comes as Andy Burnham prepares to host French President Emmanuel Macron at Downing Street for the first time since becoming prime minister.

Gang members have told our sources that others are being prepared, to the concern of emergency teams on both sides of the Channel, because of the increased risk to more lives. On 4 August 170 people had to be rescued after theirs caught fire in the middle of the Channel.

With French officers claiming they are now intercepting two-thirds of small boats before they can reach the coast, that means fewer crossing opportunities. And for those hoping to make the journey, it means a higher price.

Today, the gangs are quoting up to €2,000 (£1,700) per person. It used to be around €1,300 just a year ago. Their passengers also have to now pay in cash, to ensure commitment.

During one early low tide on Gravelines beach, we saw the only small boat to launch that day, and it was not what we expected.

At 09:30, a dozen or so migrants emerged from the sand dunes. Their distinctive orange life jackets, purchased from a local sports shop, made them easy to identify.

A need for speed at this point would be a fair assumption, but this was not to be the case. There was no running. It was almost orderly. A refined process where instructions were obeyed.

As they waded into the calm shallows of the Channel, their "taxi boat" came into view. It is simply a small boat that has departed from further along the coastline to avoid detection. Only now, they are being forced to set off from even greater distances. This one had left from Belgium.

The reason for the group not rushing was because the boat was already in the water. French police argue they are prevented from intervening at this point over fears of people drowning. It is to the long frustration of the British authorities, who would like their neighbours to at least adopt a more "case-by-case" approach.

As they clutch their phones in plastic wrapping, the group patiently waits for what they hope will be a voyage to a better life in the UK. Some tell me they are from Afghanistan. They are eager to catch their ride. Those who make it on are visibly elated.

After the last passenger, a young boy, is almost thrown on board, it inches out to sea, but it is not leaving yet.

At 09:49, another group of migrants move into the water and wait to be collected. An hour later, the boat indeed does return, but to another gathering that has emerged on a different part of the beach. A few run the vast distance across the sand to try to secure a spot. Most manage to clamber on.

As police officers watch from the shore, the small boat goes on to make five collections like this over three hours. The boarding becomes more frantic as the window of opportunity shrinks.

By 12:11, the small boat, with 82 people on board, finally moves off into the busiest shipping lane in the world. It is dwarfed by the passenger ferries in the background.

Everyone successfully made it to the UK. Small boats typically radio for help once they are in British waters, as the coastguard is compelled to rescue them and take them to shore.

What they leave behind in Gravelines is a scene of dejection and exhaustion. A limp toddler is carried past me towards a police officer, who revives him with a light shake.

This all unfolds next to four or so men casting their fishing rods into the increasingly choppy waters. It is peak holiday season here. They and other dogwalkers barely glance at the crestfallen migrants wading back to the stand.

Before, the gangs would often launch boats in the dead of night, but with increased police patrols, charity workers tell us the gangs now see the middle of the day as a better opportunity, as it takes taxi boats longer to arrive and it allows them to better co-ordinate boarding.

"You feel guilty being there - going to the beach and having a good time, while these families are in such terrible situations," observes a woman with her young daughter. They had been playing with a bucket and spade.

"It's hard," she admits. "We regularly see people on the beach attempting the crossing. It feels like living in two parallel worlds that have nothing to do with each other; it's really strange."

The migrants remaining will wait for their next opportunity, as per their agreement with the gangs. But because of more robust policing that wait is now typically weeks, even months, rather than days.

They also need a favourable weather window: the wind needs to ideally be under 5 knots and waves need to be less than 0.8m for a small boat to have a chance of making it across.

To improve their chances of avoiding detection and to be closer to a taxi boat, charity workers tell us migrants are also sleeping closer to the beach in the woods or among the dunes, instead of at the larger camps where they can wash, or receive food, water and medication.

It is after arriving at the largest settlement at Loon-Plage, near Dunkirk, that we soon find ourselves in direct confrontation with the people smugglers.

As the intense heat bears down on a tent where people can charge their devices, we initially meet two young Egyptian men who are visibly tired and disheartened. They have been staying in squalid conditions for months and have made several unsuccessful attempts to cross.

"The police stopped us from leaving and destroyed the boats," explains one. He says he wants to go to the UK so he can work and obtain legal papers.

A young Sudanese woman and Afghan man also told us they had also tried to cross dozens of times. We have been to the camp on several occasions over the past year, but this time there was a perceptible unease.

Riot police had cleared out the tents in the same week, and the day before around 400 migrants preparing for a taxi boat in the nearby Hardelot forest were forced to return after several fires broke out.

After agreeing to stop filming, one smuggler, around 5'8" tall (172cm) and in a new grey tracksuit, tries to grab our camera. He then repeatedly throws rocks at us from close range, striking our cameraman above the eye.

We're chased to the car and make it away. Pol would later receive a few stitches.

The UK has been quick to praise the French police for reducing crossing numbers to a six-year low. Officials also cite a deeper, more coordinated level of coordination between the two countries.

But changes in immigration policy, the movement of people and the weather means the broader picture is more complicated.

Thousands are still illegally entering the UK every month on small boats. The numbers remain significant. And motivations to make this journey still run deep in this stretch of northern France.

For the people smuggling gangs that still means money, and a need to evolve.

Additional reporting by Marianne Baisnée