On screen, a healthy-looking boy of 17 stands cockily, one hand on his hip. "Heroin," he says. "I dunno why there's all this fuss about it. It might be a problem for some people, but I could handle it."

The scene dissolves into others, with the boy looking progressively scruffier and sicker. As his speech deteriorates, he claims, "There's no way I'm going to become an addict."

Finally, he is sitting on the floor of an empty room, his body hunched and sweating. "Look, I've got this thing under control. I've just got a touch of the flu today. That's all." His voice is barely audible. "I could give it up tomorrow. Couldn't I?"

This is one of two television commercials -- the other featuring a girl whose face breaks out and hair frazzles from heroin use -- now being shown on programs with teen-age viewers all over Britain. Along with a series of advertisements in teen magazines like Blitz and Faces, it is part of a controversial, multimillion-dollar publicity campaign warning young Britons against heroin.

The British government has allocated nearly $20 million to combat what law-enforcement agencies call a fast-spreading drug epidemic that nearly has tripled the number of heroin users in the country since 1979.

Once limited to a small core of several hundred addicts centered in London, heroin use now has become so common in schoolyards and neighborhoods throughout the country that Prime Minister Margaret Thatcher has warned that it threatens to "undermine a whole generation."

Because the problem is perceived to be greatest among the young, the publicity campaign has been tailored for teen-agers. But the overall antidrug offensive is comprehensive, targeted at the sources of the drug and its domestic traffickers as well as its users.

British officials, who see the United States as the pacesetter both in illegal drug use and the fight against it, have consulted closely with their U.S. counterparts in planning strategy, and many elements of the program are patterned after continuing U.S. campaigns.

In a series of exchange visits across the Atlantic, "we've seen firsthand what the possibility was," said one government official here. "We were interested in how much the Americans could tell us about how to deal with a problem descending suddenly. They concentrated more on enforcement, and there is also a lot of interest here in task forces and confiscation of drug traffickers' assets.

"The United States is several years ahead of us," he said. "As often happens, in many respects, your problem always seems that much worse."

Among the measures adopted here, the government has begun stationing customs officers in countries like Pakistan that are primary sources of heroin, and contributing money for substitution of other crops for poppies abroad. The maximum penalty here for trafficking in narcotics has been increased from 14 years to life imprisonment. This fall, new laws, copied from U.S. legislation, will be introduced to loosen banking and privacy regulations to allow investigation and seizure of drug-earned assets.

Britain even has its own celebrity antidrug crusader, whose efforts are akin to those of Nancy Reagan's campaign. After Diana, the princess of Wales, appeared on a British Broadcasting Corp. television special last month urging adolescents to swear off drugs, telephone calls by anxious parents to a special drug hotline jammed national circuits.

Elements of the program have met with some disapproval, particularly among social service and law enforcement agencies, whose experience with drug smuggling and addiction predates the current wave of national concern. Customs officials have argued publicly that Thatcher's much-heralded appointment of special drug investigators has been at the expense of substantial cuts in the number of uniformed inspectors who apprehend most of the illegal drugs seized at ports and airports.

Drug treatment clinics say that the publicity and education programs have raised public fears as well as willingness among some addicts to seek help, but that Thatcher's overall clampdowns on public spending have meant that there is little help to offer. So far, the government has offered about $16 million for pump-priming of local treatment units over a three-year period. After that, communities are supposed to find money out of their own resources.

"I have to be fair to this government, and say that is the first that has handed out so much money," said Jeff Boyd, a social worker at Hackney Hospital Drug Dependency Unit in London. "But on the other hand, it's not much money, and we have a drug problem of immense size."

The Hackney unit is designed to serve five of London's poorest boroughs, with a combined population of 1 million. Located in three basement rooms next to the hospital laundry, it currently is staffed by one psychiatrist, one social worker and a receptionist, although staff expansions are planned.

Ideally, Boyd said, anyone requesting treatment should be seen and placed in a program within eight days. "This morning," he said in a recent interview, "I had to say to somebody, 'Well, we have a hell of a waiting list. I can't give you an appointment. The receptionist will write to you within two weeks to a month.' "

Most of those turned away, he said, do not come back.

Government officials are prepared to acknowledge that Britain's 13.5 percent unemployment rate has been a factor in the the spread of drug abuse. But while heroin addiction has grown by quantum leaps in poor areas such as Hackney and the city of Liverpool, it is by no means limited to those in the lower economic strata.

"You have to equate it with depressed areas, but there are a lot of other variables," including Britain's historically relaxed attitude toward drugs, said an official from the Home Office, the government department in charge of domestic matters including law enforcement and drug problems. "There is evidence that availability is important, too."

The government dates the beginning of the heroin epidemic to coincide with the fall of the shah of Iran in the late 1970s, when many upper-class Iranians put their savings into easily salable heroin and fled to Britain.

"Suddenly, we had an enormous flooding of heroin, which we didn't have before," the official said. With the Iranians came the practice of "chasing the dragon," street jargon here for smoking, rather than injecting, heroin in the mistaken belief that inhaling the vapors is nonaddictive.

The wave of Iranian heroin was followed by a bigger flood from Pakistan, where the Soviet invasion of neighboring Afghanistan in late 1979 and domestic political upheaval contributed to a reordering of supply routes. Until the late 1970s, the demand for heroin was small in Britain, and its ports served primarily as transit points for drugs heading to other destinations.

A vicious circle began, as the growing number of users encouraged an increased supply, which in turn promoted an expansion of the market to more and ever younger users. Almost overnight, it seemed, parents throughout Britain began to find charred pieces of foil -- debris of users -- under the beds of teen-agers who had turned listless.

Despite evidence of the growing habit among youngsters, a special government advisory committee argued against an antidrug publicity campaign, saying it would "stimulate and encourage interest" in drugs in general. Research by the Home Office among parents, professionals and youths, however, led to the conclusion that teen-agers "already were very aware of drugs, but often not very well-informed," the official said.

In overruling the advisory group, the Home Office went to Yellowhammer, London's trendiest youth-market advertising agency, to commission a series of advertisements designed to warn young people against heroin.

After doing some research of its own, Yellowhammer decided that what Britain did not need was a campaign patterned on U.S. antidrug advertisements, using "scare tactics" like the threat of imprisonment. Teen-agers, said Yellowhammer director Sammy Harari, "see people around them smoking heroin, exhibiting all the positive aspects of it and none of the negative ones. If you say, 'Take heroin and you'll become a junkie and die,' they won't believe it.

"Among the other theories we developed," he continued, "is that there is no such thing as a pusher" to warn against. The person who introduces most youths to heroin usually "is a friend. And it is free."

Harari traveled to Liverpool to interview inner-city youths and found that "kids are very sophisticated, but they are illiterate . . . and completely inarticulate. Long words like, 'Heroin will make you impotent,' didn't work. They didn't know the word."

The message, he decided, would have to be one to which young people would relate. The advertisements focus on the "sliding slope" of heroin usage, from initial apparent harmlessness to eventual problems that are not easily avoided even by the most careful user -- from skin breakouts to sickness to addiction.

In searching for the correct messenger, Harari rejected rock stars, because "kids simply don't believe that a pop star tells them not to take drugs, and then doesn't walk around the corner and do two lines of coke."

Similarly, he discarded the Princess Diana approach. The hotline television program, he said, "was appalling. It is aimed at the middle class Brit, and it is just as likely to have the kids saying, 'Great, I think I'll go out and do it' . . . . Having Princess Di saying don't take drugs is about as realistic as having Nancy Reagan saying don't take drugs."

The completed print and video advertisements were presented to the government with some trepidation by Yellowhammer, Harari said. "Originally, we suspected that the whole campaign was designed for vote-getting" among Thatcher's white, conservative constituency.

"We told the government they had to accept a couple of things" -- that the advertisements would be placed 'underground' on teen programs and in magazines where they would be invisible to the average parent or voter. And, Harari acknowledged, in the process of focusing on the dangers of heroin, the advertisements "imply that soft drugs" like marijuana "are not that bad."

Even the principal ad slogan was bound to raise conservative hackles. "Time and time again kids said to us, 'If you really want to tell kids about it, you should say "Heroin Expletive You Up."' We knew the government wouldn't even like 'screw'," Harari said. But "we tried out things like 'Heroin is a Pain' and 'Say No to Heroin.' 'Heroin Screws You Up' was head and shoulders above."

Lower-level bureaucrats balked, so Harari took it to top government ministers. The campaign was approved, and the advertisements began appearing this spring. In the fall, surveys will be conducted to gauge the response.

"I don't think the government has gone into this fully convinced of its effectiveness," said the Home Office official. "But it does no harm."