‘If we’re all going to die, it’s better we all go down together,” insisted my grandad to his wife and eight children, before embarking on a journey that would plunge them all into a paralysing unknown.

In 1979, my family were forced from their home in Saigon, which had fallen to the North Vietnamese communists and been renamed Ho Chi Minh City. Their land, business and money were seized. “We were literally thrown out into the street,” my uncle says. Facing persecution and incarceration in one of the severe “re-education” camps, they decided to join the many thousands of “boat people” – Vietnamese refugees who fled from communism by sea.

Like me, Rachel Nguyen was born in Britain to boat people. She takes a personal journey of discovery in the powerful final episode of A Very British History, a four-part series exploring migration to Britain in the 20th century, which is broadcast tonight on BBC Four. Nguyen renders a harrowing vision of the tragedies of wartime and interviews boat people and their family members about their beginnings in Britain in the 70s and 80s. “They had no concept of what life was going to be like or what Britain would look like,” she says at the outset of the story. “I think it was a huge culture shock for them.”

Nguyen is a therapist specialising in trauma; she brings a necessary sensitivity to the lifelong burden of surviving such violence and upheaval. Post-traumatic stress disorder and other mental health problems are common among refugees, and Nguyen shows how forced migration affected older generations and those who grew up in Britain.

Facebook Twitter Pinterest ‘A necessary sensitivity’ ... Rachel Nguyen, host of A Very British History: Whatever Happened to the Boat People? Photograph: Ed Barlow/BBC

Not all of the boat people’s experiences are the same, however. Some are haunted not only by ghosts of the past, but also by the living. My uncle remembers meeting a victim of Thai pirates, who were responsible for capturing, raping and killing thousands of boat people. She had been dragged on to their ship and raped over three days, before being thrown overboard. Suppressed below the surface, these memories risk being forgotten. Nguyen’s open discussion with the boat people community is a talking cure, where storytelling can help them cope with and piece together a broken narrative.

Watching the programme made me reflect not only on the journeys of the boat people, but also on the conditions that led them to leave in the first place – and the tough decision my grandad made amid the chaotic siege of Saigon. His daughter, my aunt Kim, had just been born and was small enough to fit inside a rice-paddy hat. Fearing that she would not survive the treacherous crossing to Hong Kong, he agonised over whether to leave her behind – under the care of his eldest, who volunteered to stay – or to take everyone. There was no guarantee that anyone would survive the journey – drowning, storms and pirates were among the many dangers – and no one had any idea where they would end up if they did.

But, in his heart, my grandad wanted his whole family to venture towards the same fate – whatever it might be. So he decided on the latter course of action, paying the hefty fee of 12 gold pieces per person to escape (a crushing sum for a family of 10). “If I die, I want you to eat my flesh,” he told his starving children onboard – a gesture that, my uncle tells me, hovered somewhere between a joke and utter seriousness.

Even though I was born in the UK, the memory of that visceral love – expressed in sacrifice – has settled deep in my bones. I knew the phrase boat people, but only recently did I begin to understand what it really meant. I belong to the same line as the people who arrived here with nothing but their stories. I grew up with those emotions, intimacies and the turmoil of doubleness – of straddling polar-opposite cultures.

Facebook Twitter Pinterest Bumpy landing ... refugees from Vietnam arrive in Britain in 1979. Photograph: John Minihan/Evening Standard/Getty Images

Although Margaret Thatcher eventually granted asylum to 10,000 Vietnamese refugees in 1979, initially she refused them entry to Britain. As documents in the National Archives reveal, she had urged ministers to decline any admissions after the first boatload, fearing uneasy assimilation. Even after accepting more refugees, the government “dispersed” them across the country to avoid concentrated communities.

Settling in small towns was an extremely lonely experience for many Vietnamese people. Racist hostility was common – but there were examples of boundless kindness, too. Very few refugees could speak any English and found only unstable work – far below the professions they once occupied. “We were made invisible,” my uncle says. My family’s arrival in Northampton sparked “curiosity” among the locals, who had seen only war-worn images of Vietnam on TV. The newcomers knew “nothing” about Britain. “The image I had of an English gentleman was a man in a black suit with a bowler hat, a briefcase and an umbrella under his arm,” recalls one of Nguyen’s interviewees.

However, there was solace and kindness, too. “I’m very happy to have come to England. People helped us so much,” says my grandma, 40 years after making the brave leap from her home. It takes sitting down and spending time with loved ones, as Nguyen does, to paint a nuanced picture of the boat peoples’ experience – one that captures the grief-suffused loss of their homeland and their successes in new communities.

A Very British History: Whatever Happened to the Boat People? is on BBC Four on 4 March at 9pm