It is tempting - too tempting - to play the game of "what might have been", or to feel nostalgic for something that never happened. A vast new box set from the British Film Institute, Land of Promise, which collects the most notable films of documentary-makers working between 1930 and 1950, is a compendium of what-ifs, in which the idea of a fair and equal Britain, one brought about by war but created in peace, seems so real and near as to feel graspable. In these richer but once more unequal times, it is hard to avoid nostalgia.
The proselytising tone of the British documentary movement was set by John Grierson, a member of the Empire Marketing Board and later founder of the General Post Office film unit. He served as producer on many of these films collated and beautifully restored by the BFI, but his true role was as a mentor and evangelist for documentary film as a tool of social democracy. The movement reached its ideological peak towards the end of the Second World War, when the prospect of monumental change was brought giddyingly close by the Beveridge report.
In Humphrey Jennings's A Diary for Timothy, made in the closing months of the war, E M Forster's emotive script addresses a real baby, born on the fifth anniversary of 3 September 1939. Timothy, "born in a nursing home near Oxford, very comfortable", is a British, middle-class, and therefore extremely lucky, war baby: "If you'd been born in wartime Holland or Poland, or in a Liverpool or Glasgow slum, this would be a very different picture. All the same, you're in danger. You're in danger, Tim!"
The war against fascism was all but won, but the fight to remake British society by slaying the "five giants" of illness, ignorance, disease, squalor and want was at its very beginning. When Goronwy, a Welsh miner featured in the film whose working conditions embody the need for root-and-branch postwar social change, breaks his arm during a shift, he is seen groaning on a stretcher as his comrades carry him to the doctor, from whom he immediately requests "a fag".
The narrator adds, gloomily: "It's pretty shocking, isn't it, that this sort of thing should still happen every day, though we've been cutting coal for 500 years. Something else for you to think of." The inference is that it's all up to Timothy's generation, who would come to be known as the baby boomers and would now be regarded variously as the most idealistic but also the least mature, and the most greedy, generation so far. "Are you going to have greed for money and power oust decency from the world as they have in the past? Or are you going to make the world a different place - you and all the other babies?"
It's not surprising that the parents of Timothy and those children born around the same time might have indulged their offspring in the desire to ensure that their postwar lives bore as little resemblance as possible to those of the pre-war years. Goronwy's children would be discouraged vigorously from following him down the pit, and would form a large caucus of working-class grammar-school children from South Wales to enter university and the professions in the 1950s and 1960s. Boys of Timothy's background would enter the BBC and ultimately strip it of the ascetic Calvinism shared by Grierson and the corporation's founder, John Reith.
Paul Rotha's Land of Promise (1946), the longest and arguably the most ambitious piece in this collection, is, like A Diary for Timothy, pitched firmly at the heart and soul, yet is replete with persuasive facts detailing the disastrous effects of speculative property-building on the interwar supply of affordable rented housing. John Mills, as narrator, is the rational counterpoint to a chorus of complacent and defeated archetypes who refuse to look the disgrace of bad housing, and the people forced by circumstance to live in it, in the eye.
It was by no means the first film to set its camera squarely on squalor in the hope of shocking those with the power and money to banish it into action. Housing Problems (1935) was sponsored by the Gas Light and Coke Company, which had just commissioned Kensal House, an optimistic but ultimately doomed experiment in Czech-style communal living in London, from the architects Maxwell Fry and Elizabeth Denby.
One might, with a degree of cynicism, say that even in these films, which contain uncomfortably lingering shots of slum conditions - falling-in ceilings, bugs, rats, walls so wet you could scoop chunks out of them - the film-makers were motivated by righteous indignation rather than rage. That is not to say their intentions - no less than to change, or at least influence, the future course of Britain - weren't honourable or important.
Beyond housing, the quality and purpose of people's lives, as shown by these films, ebb and flow with economic demand, which was exactly the flaw in capitalism that thinkers on the left (Grierson's documentarists being among them) sought to banish after the war through centralised planning. In the Barrow-in-Furness of Rotha's 1935 film Shipyard, where the men socialise by racing dogs (who are shown being caught with a rag and swung around affectionately like football rattles), "the life of the town is the work of the yard" - inevitable when ships-in-progress can be seen from all points.
That one phrase captures the literal sense of redundancy felt in such towns when work disappears, a premonition of which is shown in the fascinating Today We Live, from 1937, which focuses on out-of-work Rhondda miners building their own community centre to keep themselves off the streets. But ten years later - as Cotton Come Back (1946) and Five Towns (1947) show - the demand for energy and labour was never higher, putting many mines back in commission for several decades and creating the full employment that would keep Britain buoyant until the 1970s.
You notice the ways in which social and economic change has effected cultural change: the way in which "ordinary people" speak in these films is either far more clipped and modulated than you would expect, or is intensely specific to region. The 1945 film Fenlands, about farmers working East Anglian land dredged by the Dutch 300 years previously, is beautiful as much for its undulating narration as for its rippled scenery. The west Cumbrian narrator of Rotha's Shipyard is the aural dead spit of Melvyn Bragg.
In the black-and-white shots that are layered throughout the booklet accompanying the box set, you can see young men - first-time directors such as Harry Watt (who made 1940's Britain at Bay, narrated by J B Priestley) and their assistants - dressed in a way that wouldn't look out of place among today's change-the-world artists. You expect to see bespectacled, bequiffed men in double-breasted suits, or at the very least a shirt and tie; but these graduates in their bomber jackets and belted baggy trousers would have looked like privileged scruffs to their working-class subjects, and would still do so today.
That is the classic paradox of these films: they are made for and about working-class people, but not by them. (That partial revolution was to come later.) Rotha, for instance, was educated at Highgate School, John Betjeman's alma mater, and the Slade School of Fine Art, and changed his name from the less exotic-sounding Thompson. What they did do, however, was help to create a space for the later social-realist, or kitchen-sink, dramas to flourish. You wouldn't have the scenes of rubbly quarries and bombed-out streets in Leeds that fill 1963's Billy Liar, for instance, if the documentarists hadn't paved the way.
The writer Ian Jack wrote last year that "the documentary is a confection and often built on a series of small lies", an accusation that strikes harder at this collection, given that it was Grierson himself who coined the neologism "documentary". There is no doubt that he and the other film-makers whose work is showcased here had an agenda; it is also true in most cases that they were working as an arm of the government or another interested party.
But you have to allow for their straining for truth by packing their work with truisms. None of these films is authentic cinéma vérité - least of all the fantastic, partly animated Chasing the Blues, about improving mill conditions - but they were filmed in real towns, using real problems and concerns as a backdrop. All are shameless - though dignified - appeals to our better selves.
"Land of Promise: the British Documentary Movement (1930-1950)" is released on 28 April (£34.99) .