Three weeks ago 14-year-old Liverpool schoolboy Jason Fitzsimmons died from an overdose of heroin he had bought from pushers within a five-minute walk of his home.

At a nationally publicized inquest, the county coroner said Jason was the youngest heroin victim he had seen in nearly two decades on the job. He spoke movingly of "problems of the sort which were completely unknown in my boyhood," and said Jason's death was part of a drug abuse problem that could endanger "the whole future of the nation."

Barely 10 years ago, the illegal sale and use of drugs in this country was practically unheard of outside a few neighborhoods in London's West End. Britain declared itself largely immune to the wave of drug smuggling and abuse in the United States, attributed by many here to the prevalence of violence and organized crime across the Atlantic.

Yet over the past five years, with a speed that has shocked British parents and apparently caught the government initially unawares, Britain has found itself with a massive drug problem that a special multiparty parliamentary study recently called "the most serious peacetime threat to our national well-being."

Throughout England and much of Scotland, illegal drug use has doubled, tripled and in some cases quadrupled since 1980. Even teen-age glue-sniffing, a fad among U.S. adolescents in the 1970s, has become so widespread here that a new law went into effect last week limiting the sniffable solvents that can be sold to those under 18.

While marijuana smoking is on the upswing, it is considered a relatively minor problem compared to the "flood" of cocaine -- previously rare in Britain -- expected to hit these shores in the next two to five years as illegal production surpasses the demand of the U.S. market. British officials have begun close consultations with the U.S. Drug Enforcement Administration and there have been frequent, high-level official exchanges.

"We make no apology for reverting repeatedly to the American experience," said Edward Gardner, head of the special parliamentary study, after leading his group last spring on a 10-day tour of the drug dens of New York City's Lower East Side and smuggling centers on Florida's east coast, "because it is in the U.S.A. that the real battle is being waged." 'Drug of Choice'

But the most serious concern here centers on the spread among young people of a drug whose use has been relatively isolated in the United States. Britain's "drug of choice," according to law enforcement and social workers, is heroin.

The government estimates that addiction to opiates, primarily heroin, has grown more than threefold since 1979, with a major surge among schoolchildren who heat the drug and inhale its vapors -- a practice known as "chasing the dragon" -- in the mistaken belief that this is less addictive than injection.

Unlike in the United States, where the high price and low level of purity make for a relatively limited market, heroin here often is cheaper than marijuana. A bag of 40 percent pure heroin that will last an addict a day or two can cost less than $15 on the street.

That figure has not changed for several years, a fact that officials attribute to an upsurge in smuggled supply. Seizures of heroin by customs officials reached nearly 500 pounds for the first seven months of 1984, more than six times the total for all of 1980. Customs officials say the boost in seizures reflects increased smuggling rather than more efficiency on their part, and estimate that no more than 10 percent to 20 percent of total imports is stopped.

There is widespread disagreement here over the reason for the jump in drug usage. Some blame the increased supply on a growing demand induced by unemployment, social malaise and a general unraveling of social mores that previously had limited drug abuse here. Others say the demand is the criminally nurtured result of a vastly expanded smuggling network from Pakistan, Iran and South America that has been rerouted to Britain in response to political events in those countries or enforcement crackdowns in the United States. A Political Football

As citizens express their sense of panic and the government of Prime Minister Margeret Thatcher attempts to get the situation in hand, the drug problem has become a political football.

Earlier this month, Thatcher paid a "surprise" visit, with television cameras in tow, to customs inspectors at London's Heathrow Airport. Examining confiscated smuggling devices, she promised that traffickers would be "pursued relentlessly" and that "we shall make your life not worth living."

"The taxpayer will find the money," no matter how much it takes to stop the trafficking, Thatcher said. "We have to beat the drug smugglers. They can undermine a whole generation and corrupt everything."

At their own news conference last week, however, unions representing customs officers charged that Thatcher had turned Britain into "open house for smugglers" with a loss of nearly 1,000 uniformed customs job slots as part of her government's drive to cut public sector spending.

Similarly, local government bodies have accused Thatcher of fanning fears among white, middle-class supporters of her Conservative Party to boost her law-and-order image, while she limits local revenues available to deal with the drugs menace on a long-term basis.

Away from the political gamesmanship, a critical examination is under way among drug treatment professionals of whether Britain's historically laissez-faire attitude toward narcotics usage helped lay the foundation for today's difficulties.

Until relatively recently, addiction was considered a problem for physicians rather than law enforcement, to the extent that it was considered a problem at all. Under a unique program known by U.S. officials as "The British System," recognized treatment for decades has included the "maintenance" of "registered" heroin addicts with government-supplied drugs.

Although the program now is severely limited and is being phased out, British officials visiting the United States in search of a solution to the new drug epidemic say the stigma of the maintenance treatment method is proving difficult to shake.

"Everywhere we went, even in Congress, people were totally convinced that we fed addicts," said Robin Corbett, a member of last spring's parliamentary tour.

So sensitive is the British government to this that it has published a four-page explanatory pamphlet entitled "The Medical Use of Opioids in the United Kingdom."

"For many years," it begins, "there has been considerable interest in the U.K. approach to the treatment of drug addicts. Unfortunately this interest has all too often been based on misunderstanding . . . . The popular view of what has become known as 'The British System' is perhaps best conveyed by its alternative title, 'The British Heroin Maintenance Program,' under which, so it is believed, addicts are 'registered' and thereafter entitled, often indefinitely, to a regular supply of heroin on a doctor's prescription. The reality is very different." Three Addicts' Stories

For more than a decade now -- nearly 20 years in one case -- Michael, Anne and Brian have begun each morning with a trip to their local pharmacy for a prescribed, daily fix of government-supplied heroin. Back at home, they inject it into their arms with government-supplied needles and prepare to face the day.

It has been seven or eight years since the last new patient was placed on an indefinite program of addiction maintenance with injectable heroin. As in many other western countries, thousands of British heroin addicts now participate in treatment programs consisting of oral doses of liquid methadone, a synthetic heroin substitute, that is gradually reduced over several months in hopes of final withdrawal and cure.

But Michael, Anne and Brian are among the fewer than 200 leftovers from an earlier era when Britain considered the potential development of a black market in narcotics a greater danger than the government-assisted lifetime addiction of a relatively few misguided individuals.

The three have asked that their names be changed so that relatives and friends who do not know of their addiction will not recognize them. According to the social worker at the London clinic where they are registered, their cases are "pathetic."

Michael is tall and thin, a neatly dressed man of 40. He became addicted "during the flower-power days of the 1960s" when, after dabbling in softer drugs, a friend with a legal prescription offered to share some of his heroin. Like Anne, who was turned on by a boyfriend a few years before Michael, he says, "I don't remember the first time being so wonderful. I got sick. I vomited."

Neither Michael nor Anne, with rare and long-ago exceptions, has ever procured or used illegal heroin. At the time they became addicted, any physician in Britain could write a legal medical prescription for the drug.

Heroin maintenance had been a medically accepted treatment here since 1926, when a government report recommended it for hard-core addicts deemed incapable of withdrawal. The reasoning was that if there was no illegal demand for heroin, no illegal heroin market would develop.

With the social acceptability of drug abuse limited to what was considered a small, avant-garde London core, "to some extent the system worked right into the '60s," said David Turner, director of Britain's Standing Council on Drug Abuse. "There wasn't a criminal element, there wasn't a black market . . . and there were maybe 400 to 500 people" under treatment.

By 1965, however, a slight increase in the number of new addicts was noted, and a government report acknowledged that some physicians were overprescribing, with the excess being sold by patients on the street. Rather than change the treatment method, the solution was to regulate the number of physicians who could prescribe heroin. In 1968, the number was reduced from thousands to a few hundred doctors licensed to operate in a new chain of public Drug Dependency Units. These units, along with the addicts, were located almost exclusively in London. %Six-Week 'Scrip'

Michael and Anne checked into the clinic, were given psychiatric and physical examinations and issued a "scrip" that is reissued to this day at six-week intervals. It is sent directly to their local pharmacist, who doles them out a daily ration of liquid heroin.

Neither of them has held a job for most of the past two decades, and both live in public housing and receive welfare benefits. They say they are satisfied with their lives, living quietly and not associating with other long-term addicts or what they call the "rubbish" among the new addict generation. "I'm not even sure what 'chasing the dragon' is," Michael says.

They are adamant about the fact that they do not "get stoned" using their day's ration of heroin at once or saving it for a binge. They consider it as similar to a diabetic's insulin, a daily medication they need to function normally.

"There comes a point when you tend to accept that, for better or worse, this is how your life has ended up," Michael says. "It's been too long, and I can't face what life would be like without it." Without the "scrip," he says he would consider suicide. Anne, whose thin arms are badly scarred, says she would be out on the street, or "throwing bricks" through pharmacy windows to get the drug.

But while the maintenance system worked after a fashion for people like them, the free dispensing of heroin prescriptions had a longer-lasting effect that the clinic system could not eliminate.

"The U.K. drug problem, without exception, started from legally manufactured, legally prescribed drugs," Turner said in an interview. "It is true to some degree that the elimination of licit drugs led to illicit supply." Tightening the Rules

Shortly after the clinic system was established, treatment agencies began to be required to report all known addicts to the authorities. As the rules tightened in the early 1970s, a small black market began to thrive within London's Chinese community, centering in Gerrard Street in the West End, and catering to those who had been introduced to legal heroin but found their continuing access restricted. It was during this period that Brian, 33, was turned on by a friend who held a legal prescription.

As his habit grew, and his performance on his regular job deteriorated, he became a dealer for the Chinese and helped turn on new users. It was not until he was arrested that he agreed to register at the government clinic.

"The reaction wasn't exactly what I thought I'd get," he said. "They already knew my name . . . other people had told them who they'd got it from. It took me a while to convince them that I actually wanted to stop dealing and get off the street. Plus, I wanted to try and insure at least in the beginning that I would get enough so I wouldn't need to continue going out on the street."

Although originally admitted on a dosage reduction basis, Brian eventually ended up on maintenance. Until a few years ago, when he began a relationship with a nonuser that led to marriage, he supplemented his clinic supply with regular black-market purchases.

Michael and Anne expect their addiction will last for the rest of their lives, while Brian says he eventually would like to be drug-free. He estimates he will not be ready for that for another three to five years.

All three are concerned that current "hysteria" about the new drug wave might lead to their program becoming a scapegoat for popular outrage. But they acknowledge that times have changed drastically -- in terms of source, supply and patterns of usage -- since their habits began.

Whichever came first -- the demand that developed during the years of relaxed attitudes toward drug taking and suddenly became an epidemic, or the vast supply that now feeds it from new sources in the outposts of Britain's former empire -- it is a problem that the Thatcher government has pledged to deal with.