TWENTY YEARS AGO, when Anthony Sampson's original Anatomy of Britain became the authoritative guide to that nation's postwar power structure and post-imperial struggle to adapt to a changed world role, it was full of the hopes of British leaders for reform and modernization of the country's aged institutions, obsolescent economy and divisive class system.

Now, just months before an important national election in Britain, Sampson's fourth and most extensively rewritten Anatomy is filled with the "failure" of those "bold promises of change and reform." Britain's leaders "had zealously set about trying to reform almost every area of administration to make Britain more efficient and equitable," Sampson writes. "Yet nearly all of these bold endeavors had ended in disillusion, if not fiasco."

In The Changing Anatomy of Britain, Sampson maintains his tradition of presenting encyclopedic analyses of Britain's institutions and thumbnail sketches of the people who run them. It is disappointing that he purposely chooses to ignore "the other millions at the receiving end of their decisions," thus depriving the book of any human dimension to those economic, social and racial problems so serious that many British cities erupted in 1981 in rioting which shocked Americans.

But, like its predecessors, the book remains an invaluable reference for anyone who wants to understand Britain today. And, more like his best-selling investigations of multi-national corporations and the oil, arms and banking industries, this Anatomy probes Britain's structural problems and the failure of its leaders and institutions to come to grips with them.

Sampson paints a gloomy picture. Britain's entrenched, elitist government bureacracy has remained "immobile" and resistant to change. Its education system has perpetuated class divisions despite a chaotic reorganization. Its legal system has become increasing irrevelant to the real problems and concerns of citizens. Its unwieldy large industries have failed to modernize and rationalize themselves sufficiently to compete in the new world of high technology. Its internationally preoccupied financiers have remained aloof from their own nation's severe economic problems. Its military has commanded too large a share of resources in a climate of imperial nostalgia aggravated by the Falkland Islands adventure. Its political parties have become much more polarized--a departure from their long postwar period of cooperation--and are now split sharply over whether Britain should return to largely unfettered capitalism or increase government control of the economy through welfare state socialism.

Over and over again, Sampson warns Britain's leaders, voters and friends that they are not facing up to the magnitude of these problems. "Throughout these twenty years the more serious politicians have realized the unique dangers of a nation in the aftermath of empire," he writes. "They knew the parallels with other post-imperial nations from Egypt to Spain, though they have kept their real fears private."

To emphasize his point, Sampson repeatedly compares Britain with doddering Imperial Spain. He even fits the Falklands war into this pattern, calling it "a quixotic enterprise, in the literal sense; for Don Quixote--as Cervantes created him in the twilight of the Spanish empire --was fascinated by the romances and past glories of Imperial Spain, embarking on his excursions of honor and principle to rescue damsels or tilt at windmills, with a romanticism which baffled the rest of the world.

"The general British condition already had some worrying resemblances to the symptoms of decline in Imperial Spain-- the preoccupation with the past and academic pursuits, the obsession with honors, the contempt for workaday trade."

Sampson, who outspokenly opposed Britain's prosecution of the Falklands war, fearing it could end in disaster, now acknowledges the efficiency with which victory was won. But he remains unconvinced of claims that it demonstrated a turning point for postwar Britain, finding few signs of similar purposefulness in tackling the country's political, economic and social problems. "The sense of purpose which Britain had shown in the Falklands expedition had no lasting value," he argues, "unless it could be transferred to the more critical industrial and social challenges at home.

Similarly, while recognizing that "Britain has some advantages which others envy, above all a tradition of political stability which looks more precious to countries afflicted by terrorism" and "ancient institutions which in the past have survived and adapted to new challenges and threats," Sampson warns that these same institutions now pose possibly insurmountable obstacles to needed change.

He surprisingly admits to some admiration for Conservative Prime Minister Margaret Thatcher's determination to force them to change, despite his strong aversion to her refusal to spend government money to ameliorate the pain, including a 14 percent unemployment rate, caused by her survival-of-the-fittest economic policies. "No one concerned by the inertia and irresponsibility of institutions could totally withhold admiration for Mrs. Thatcher, who insisted on the responsibility of the individual and who tried much harder than her predecessors to cut down the Whitehall bureaucracy and stimulate competition," Sampson admits. "No institution looked quite the same after she had cast her beady eye over it."

But as Britain approaches an election that could give Thatcher five more years to pursue her particular crusade for change, Sampson makes clear he sees her as aathrowback to an earlier era that can never really be relived. "Any cure to the British post-industrial malaise . . . will not be achieved by trying to recapture the self-confidence and arrogance of imperial Britain," he argues, "which was based on a military power and industrial supremacy which can never return."

Instead, Sampson betrays his behind- the-scenes role in the new Social Democratic Party by advocating its centrist alternatives to Britain's polarized major parties, class conflicts, labor-management conflicts and unchanging institutions. He fails to ask himself whether his concluding, characteristically Social Democratic prescription for Britain's future does not sound like those unfulfilled aspirations of 20 years ago: "It will depend on new opportunities, new kinds of people outside the old classes, who are aware of new opportunities, innovations and new markets abroad."

If this sounds like the new-everything pronouncements of several Democratic presidential candidates, if Margaret Thatcher sounds much like Ronald Reagan, and if American voters appear likely to face in the presidential election of 1984 some of the same choices (minus a radically socialist major party on the left) that the British will have when Thatcher calls a national election sometime within the next 10 months, it makes Sampson's point. His new anatomy of Britain "has an epic theme which is more universal," as similar "problems of retreat and adjustment now also face other nations including the United States."

This adds to the importance of a book that comes closer than any other to a clear-headed analysis of what can no longer be called only "the British disease." Sampson offers object lessons for readers on this side of the Atlantic as well.