About halfway through our interview, I call Ken Loach a sadist. The mild-mannered, faintly mole-like film director blinks hard, chuckles, and carries on. We are discussing a key aspect of his film-making: the element of surprise. Loach has spent his career depicting ordinary people, telling working-class stories as truthfully as possible, and he works distinctively – filming each scene in order, often using non-professional actors, encouraging improvisation.

They don't tend to see a full script in advance, and move through his films as confused as the audience about what lurks around the next corner. I ask Loach which surprise was most memorable, and he laughs incongruously through a few examples. He talks about an incident when an actor walked through a door, on-set, to discover his co-star in a bath, her wrists apparently slashed. "Surprise is the hardest thing to act," says Loach, "and his response was just very true." On another occasion an actor only found out during the filming of a battle scene that her character was to be shot and killed. She was not especially pleased.

Most surprisingly of all, Crissy Rock, the lead in Ladybird, Ladybird (1994) – a brilliant, devastating gut-wrencher of a film – was convinced she was starring in a happy, upbeat, redemptive story. "She thought it would turn out to be about a couple successfully raising children together," says Loach, smiling. It is actually about a woman's kids being taken, one by one, by the social services. In the scene where they come for the final child, Rock "couldn't believe it," says Loach. "She was just wrecked."

It's at this point I laugh and call Loach a sadist. But it's probably more accurate to call him uncompromising, with both his actors and his leftwing politics. Loach turned 75 in June, and next month the BFI is showing a retrospective that will take viewers from his early television work – including the harsh, effective, 1966 exposé of homelessness Cathy Come Home – to his most recent film, Route Irish, about the experiences of private security contractors in Iraq. I ask which of his films he's most proud of, and he can't choose. "There's quite a few I cocked up, but that's another matter."

His documentary The Save the Children Film, part-funded by the charity, is being shown for the first time; made in 1969 for TV, it was never broadcast. The film was commissioned for the charity's 50th anniversary, and it's easy to imagine what they might have been expecting: a gauzy portrait, light on analysis, strong on praise.

Loach took a different tack. The documentary looks at the potential problems of aid, the ways those in a position to be charitable are often patronising and paternalistic. He took his cameras to a school run by Save the Children in Kenya, for homeless boys from Nairobi, for instance, that was set up along the lines of a British public school; the children are shown blowing bugles, marching, reading books including The Inimitable Jeeves and Tom Brown's Schooldays. A group of young Kenyan activists appear in the film, one of whom notes he can't think of another school in the world where the mother tongue isn't allowed.

The documentary moves beyond the charity's work to show British expatriates in Kenya; one stompingly posh woman remarks they have "a wildly gay time" there, and she feels that "even in their poverty, [the Kenyan people] are basically happy". Raising their living standards might just upset things, she adds. The film is full of issues that remain pressing: the limits of philanthropy, the patronage relationships fostered by aid, the subtle and not-so-subtle problems of colonialism. It ends with the comment that we "must change the property relationships of society, and then we change man. That's the only real solution, and all the rest is propaganda."

The documentary was made for LWT and only one-third funded by the charity, so Loach thought he and his crew could "take an independent view, and the TV company would support us. But they didn't." There are moments when he seems to have the naivety that derives from an inflexible moral backbone. "When the people that ran the Save the Children Fund said they would sue us, the television company wrote off their investment, and didn't back us at all."

It isn't the only one of his documentaries to have been pulled; Questions of Leadership, a TV series critiquing the response of the trade union top brass to Thatcherism, made in the early 1980s, was never shown – apparently for political reasons. I ask whether it upsets him when his films are censored or withdrawn, and he says: "It makes you angry, not on your own behalf, but on behalf of the people whose voices weren't allowed to be heard. When you had trade unions, ordinary people, rank and file, never been on television, never been interviewed, and they're not allowed to be heard, that's scandalous. And you see it over and over again. I mean, we heard very little from the kids in the riots. You hear some people being inarticulate in a hood, but very few people were actually allowed to speak."

Loach's films are often either a call to arms (a reminder of the rotten, vicious circumstances many people face) or portraits of specific political movements. The Wind That Shakes the Barley was about the Irish war of independence, Bread and Roses was about a group of office cleaners in Los Angeles campaigning for just wages, Land and Freedom was about a young, unemployed Liverpudlian man, a member of the Communist party, who heads to Spain to fight in a militia against Franco.

Land and Freedom has all the obvious elements of a great film: a passionate protagonist, beautiful heroine, a romantic relationship, battle scenes. And yet its most gripping moments involve an extensive conversation between a communist militia and the people of a Spanish town about the merits of collectivism. Should the town's land be carved up and shared among the people? Should some remain private? All of it? When I ask whether Loach would describe his politics as socialist, he says it's a difficult word, because it's much devalued, and you can't "make sense of it without Marx – but if you say you're a Marxist, then the rightwing press just uses it as a brick to hang around your neck." He is the rare film-maker who brings questions of political structure flamingly alive.

Loach is a quiet, gentle man – that streak of sadism aside – who seems entirely without vanity; he comes across like the most caring teacher in school. We talk more about the riots, and the subsequent heavy-handedness of the courts. "They'll shoot people for stealing sheep next, won't they?" he says. "But, in a way, whenever something dramatic happens, you know that everybody retreats to their comfort zone – so the Tories retreat to cutting benefits, pulling people out of their houses, savage prison sentences. They want that anyway. So whatever happens is an excuse for them to do what they want to do."

I mention the two young men put away for four years each, after trying to provoke rioting through their Facebook pages. Loach notes, with a shrug, that their cases will probably go to appeal, then adds: "It's the ruling class cracking the whip, isn't it? It's disgusting. We've got to organise. In the words of the old American trade unionist Joe Hill: don't mourn, organise."

He continues, apologising occasionally for "lecturing" me. "I think the underlying factors regarding the riots are plain for anyone with eyes to see … It seems to me any economic structure that could give young people a future has been destroyed. Traditionally young people would be drawn into the world of work, and into groups of adults who would send the boys for a lefthanded screwdriver, or a pot of elbow grease, and so they'd be sent up in that way, but they would also learn about responsibilities, and learn a trade, and be defined by their skills. Well, they destroyed that. Thatcher destroyed that. She consciously destroyed the workforces in places like the railways, for example, and the mines, and the steelworks … so that transition from adolescence to adulthood was destroyed, consciously, and knowingly.

"I don't recall the nihilism among kids now, 40 or 50 years ago," he says. "Now there is no place for kids, period. So I think despite the material advances, we're worse off." We also don't seem to have a political class that understands, on any level, what it's like to face unemployment. "No, the Bullingdon boys have never had to confront that," says Loach. "The Bullingdon boys will wreck restaurants and …" he pauses. "Just throw some money at it?" I say. "Yes, or their parents will throw money at it."

I ask whether he aims to provoke political change with his films, and he says he hopes they make people "see things in a different way. That they see there were possibilities for change in Spain, for instance, and one of the things that destroyed it was sectarianism on the left. That you can organise trade unions, we do have strength, things can be different, and here are stories from the past that show it."

It's difficult to imagine young people risking their lives for leftwing ideals now as they once did in Spain though, isn't it? Loach disagrees. "You get the international volunteers who go and put themselves in Gaza … Those are the sort of people who would have gone to Spain. People will resist, and they will fight back, and they do feel solidarity."

Does he think there's a chance of a revolutionary moment in the UK, after the financial crisis, the MPs' expenses scandal, the phone hacking revelations, and the exposure of the cosiness between the police and the Murdoch empire? "It just needs leadership," he says. "It's like a head of steam. The steam won't drive anything unless there's an engine, and somebody to stoke it, and to drive the wheels around." The moment in recent history, he thinks, when a proper movement could have been launched, was at the march against the Iraq war in 2003.

"At the end there should have been a hundred tables, here's a pen, give us your name, we're anti-privatisation, anti-war you know – it's Lenin's bread, land and peace. If you sign up to that, you'll be organised and it'll be democratic and there will be no vain personalities trying to take it over, and we can articulate a programme and a movement that might become a party on that basis. There was a huge feeling across the country. None of the politicians spoke for us. That was the moment, but it was missed."

As a child growing up in the industrial town of Nuneaton, his paternal grandfather a miner, his father a foreman in a machine tool factory, Loach had little interest in politics. His father read the Daily Express, and was a working-class Tory, and Loach, an only child ("not because they didn't want more children, but because it wasn't possible for some reason"), fell in love with the theatre. They lived 30 miles from Stratford-upon-Avon, "so once I got the bug, aged about 12 or 13, I used to go there and see plays". He fainted once, standing at the back watching Titus Andronicus, starring Laurence Olivier and Vivien Leigh. "She has her tongue cut out, and she came on with streams of red, and I'd had this long cycle ride, so I fell over."

Loach went to the local grammar school, which took in "60 boys each year, out of a town population of about 70,000. It was very lucky to go, because it was good, but it was at the expense of hundreds of boys the same age who, from the age of 11, would have no way into higher education." If you didn't get into the grammar school, your academic career would almost certainly fade fast. Loach wanted to be a lawyer; I ask if this was out of a striving for justice, and he laughs. "By no means. I just fancied the frocks really. I was really stage-struck, but I thought that going into the theatre would be unrealistic."

After a couple of years of national service with the RAF, he studied law at Oxford, and spent all his time performing. "I didn't go to a lecture for over a year. It was an absolute disgrace. I got an actor's degree – I swotted for six weeks at the end." He and a friend came close to starting a theatre afterwards, but when their funding fell through, he ended up understudying a comedian who was playing opposite Kenneth Williams. "I was totally incompetent, so thank God I never had to go on." Williams "was quite friendly", says Loach, "but if he was not in a good frame of mind, he could destroy you, and I was a young innocent abroad."

He emphasises that he was a poor actor ("I wouldn't have employed me") and so became an assistant director at Northampton Rep for a year, then in 1963 landed a job as a trainee director at the BBC. "It was a huge stroke of luck to be there, because the BBC was in quite a liberal mood, with Hugh Carleton Greene as the director general." Loach was soon influenced by the political passion of those around him, and began reading widely about leftwing ideas.

He started off directing Z Cars, and was then asked to join the Wednesday Play; in his first year he directed about six films, "original scripts, going out at peak viewing, straight after the news, when there were only two and a half channels. So everybody watched it. It was an incredible opportunity." He worked with strong writers, including Nell Dunn, Jeremy Sandford and David Mercer; Jimmy O'Connor wrote a film about capital punishment, Three Clear Sundays. "He himself had been arrested and convicted of murder and sentenced to hang, and was reprieved with days to go. He was a very good writer."

Over four or five years, he made such classics as Up the Junction, Cathy Come Home, and his first feature film, Poor Cow, all stories of working-class life. He and his peers gravitated to these stories for a number of reasons. "One is that the drama is most intense among people who have got little to lose," he says. "They live life very vividly, and the stakes are very high if you don't have a lot of money to cushion your life. Also, because they're the front line of what we came to call the class war. Either through being workers without work, or through being exploited where they were working. And I guess for a political reason, because we felt, and I still think, that if there is to be change, it will come from below. It won't come from people who have a lot to lose, it will come from people who will have everything to gain." He pauses, and smiles. "They also have the best jokes."

Loach says that period at the BBC was "hugely intoxicating" not just because of the enormous audiences, but because the directors had to defend their work, and politics. "Not only did you get reviews, but if you had a play on, you'd go on a programme called Late Night Line-Up, and there would be a critic, and a discussion, and you'd be torn to shreds, so you had to know your stuff. We always felt we were in politics, even though we were doing drama. A lot of directors now, I notice, when people take issue with them, they say: 'Oh no, it's not political, we didn't mean that, and they back off.' Well, we never backed off, you know, and why would you?"

The BFI's Ken Loach retrospective launches at BFI Southbank on 1 September with the premiere of The Save the Children film and continues until 12 October. Ken Loach at the BBC is available on DVD from 5 September.