The vision of the English countryside as an Arcadian paradise of rosy-cheeked peasants was largely the creation of town dwellers. It is the myth of those for whom the country is a place to look at rather than live in. Thomas Hardy, one of the finest of all memorialists of rural England, knew that there were scarcely any peasants to be found there at all, if by "peasant" one means a farmer who owns and works his own land. Most of them had been reduced to landless labourers by market forces and the Enclosure Acts, or driven into the satanic mills of early industrial England. There was nothing timeless or idyllic about this landscape of capitalist landowners, grinding poverty, depopulation and a decaying artisanal class, which is one reason why Hardy is not the most cheerful of authors.

Roy Strong's vision of England, by contrast, is a dream of rural harmony. His book begins with a florid gush of praise for Elizabeth I, a woman one suspects he would dearly like to have moved in with, and takes us by way of Shakespeare, the Book of Common Prayer, Thomas Gainsborough and John Constable to Vaughan Williams, Edward Elgar, folk music and the National Trust. Not much for your average British Bangladeshi there. In an elegiac epilogue, the author laments the fact that so much history teaching today passes over this national heritage for such unpatriotic topics as the Russian revolution, the two world wars and the Holocaust. Our children are having their heads filled with Trotsky rather than the Tudors.

Strong may be a Romantic conservative, but he is not a fascist beast. In fact, the most astonishing aspect of his book is the way it stealthily undermines its own thesis. He is chary of history lessons that focus on Rommel rather than Rupert Brooke, but admits that his own education was excessively Anglocentric. He also welcomes multiculturalism, even if he would clearly like Pakistanis to learn all about manor houses and morris dancing. Nor is he far from confessing that the proud lineage of British Protestantism might more accurately be described as visceral anti-Catholicism. Despite an anodyne reference to the "heroic voyages" of such squalid adventurers as Drake and Raleigh, he is well aware of the role played by imperialism in the formation of British identity. He also notes that one of the keynotes of that identity was a sense of "external threat", a polite way of describing a pathological racist revulsion at all things French. Without the French to execrate, the British would have had no more idea of who they were than a spaniel.

Most astonishingly of all, Strong sails close to acknowledging that the subject at the heart of his book is all a con. It is an "invented paradise", which glosses over the social inequalities and "appalling depression" of rural England. The great 18th-century landscape painters may show the landowner gazing benignly on his flocks of sheep and abundant harvest, but Strong reminds us that there is no sign of those who actually till the soil. (He might have added that much the same is true of Jane Austen, who has a remarkably astute eye for the value and size of a landed estate, but never sees anyone working there.) Paintings that romanticised rural life, the book points out, were displayed on the walls of some of the very landowners who, by mechanising and enclosing the countryside, were helping to destroy the communities these images commemorated. The former director of the Victoria & Albert museum seems to teeter on the very brink of Marxist talk about social contradictions.

The pastoral, in short, is the invention of patricians. Nostalgia can serve the purposes of the hard-headed, as with today's heritage industry. Because rural landscapes do not change all that much, Box Hill being pretty much the same place today as when Austen wrote about it, it is easy to imagine that the way of life which goes on in their midst is eternal as well. Strong is alert to this mistake, as well as to just how much these dewy-eyed icons of England exclude. The industrial revolution is written out of the nation's self-image, and along with it more or less everything north of Warwickshire. Englishness is more a matter of the south Downs than the Yorkshire moors. Our sense of the nation is the imaginary construct of poets, Strong concedes, not a reflection of reality.

The idea of a timeless rural England emerged at exactly the point when the country was becoming the first in the world to undergo industrialisation. The countryside, in short, became changeless just when it started to shrink. A couple of centuries earlier, when Shakespeare's John of Gaunt delivered his famous eulogy to "this scepter'd isle"', he was, so Strong points out, lamenting the passing of an England which was in fact being defined for the first time at that historical moment. As the critic Raymond Williams was fond of insisting, the only sure thing about the organic society is that it has always gone.

What is truly stunning about the book is that none of this in the end is allowed to count against the delusion of England as Arcadia. It may leave out rather a lot, but that is the way with myths. The exclusion of "factories and furnaces, slums and human degradation" from the nation's sense of selfhood is no great problem, since all such images are works of the imagination. One wonders whether Strong has ever heard of Dickens and Joyce, those mighty mythologists of the city. Why should the imagination not draw its inspiration from diesel engines as much as from haylofts? For all its bogusness, much of it well examined here, this preposterous vision of English society must be allowed to stand. It speaks, Strong informs us, of a "peaceful and tranquil" society, "that exists in harmony and where life follows the cycle of the seasons". Whenever one hears talk of social harmony, one can be sure that someone's interests are under threat. The fact that England is neither tranquil nor harmonious, and never has been, is no objection at all to this fantasy. Ideologies are not to be loused up with the facts. The rural vision remains the key source of the nation's identity.

There is, perversely, something to be said for Strong's decision to persist in this neo-Georgian delusion. Ruling-class English ideology has always involved a curious mixture of the rural and the imperial  which is to say, the peaceful and the bellicose, or even, stereotypically speaking, the feminine and masculine. Strong plumps for the former partly because he dislikes the idea of a national identity bloated with imperial mythology. As a spiritual aristocrat, he has no truck with the complacent notions of progress and conquest of a Whiggish middle class.

What this overlooks is the fact that these two visions are really sides of the same coin. The common soldiers who fought in the two world wars, Strong argues, may have come from factories and offices, but they fought in the name of Chipping Campden and Lavenham rather than Manchester or Birmingham. This isn't actually true. The great majority of these men would never have heard of Lavenham and fought because they were forced to. Yet in a garbled way, the point captures a kind of truth. Visions of peace and harmony are the agreeable illusions of which war and imperialism are the nightmarish underside. The thought of gathering lilacs in the spring again may be just enough to sustain you through Flanders or Dunkirk. In this sense, all utopia has an element of truth. When Strong writes of the English love of gardens in this book, he sees that there may be some race memory at work here, by which the citizens of post-rural England may find a fragile link back to the agricultural past of their ancestors. Not all nostalgia is morbid. But neither are we defined by the past, as this book dubiously suggests.

Terry Eagleton's Why Marx Was Right is published by Yale University Press.