LONDON, AUG. 1 -- They come to Britain alone or in pairs, blending into local communities, taking steady jobs or attending school, cultivating a low-profile appearance of placid normality. But they see themselves as soldiers behind enemy lines -- and when a communication arrives from across the Irish Sea, they become active participants in a protracted secret war.

For the past two years, police say, a small group of Irish Republican Army operatives, perhaps only a dozen men and women, have waged an increasingly successful campaign on the British mainland, striking at soldiers and institutions with explosives and guns.

Varying their tactics and targets, they have managed to keep Britain's sophisticated security forces off balance. Two days ago, members of the IRA hit their most prominent target of the current campaign: British politician Ian Gow, one of Prime Minister Margaret Thatcher's closest friends and allies was killed by a car bomb while backing out of his driveway.

The result has been a harvest of media attention and political outrage in Britain and a morale boost for IRA members and supporters back home. Gow's death also has been a blow to British police, who are under increasing criticism for their inability to apprehend operatives who have killed 15 people and injured 60 in what is now the largest wave of attacks on the mainland since the early 1970s.

"If you look at the number and variety of terrorist attacks, it's only fair to say the police haven't been very successful," says Paul Wilkinson, director of the Research Institute for the Study of Conflict and Terrorism here. "We need to look again and ask ourselves some very basic questions on what we've been doing wrong and how we might improve if we're going to catch these people."

Police and security agencies involved in the manhunt are being accused of failing to adequately coordinate their intelligence and operations. Some critics also are blaming authorities in Ireland for inadequate surveillance and enforcement against IRA groups operating beyond the reach of British police.

Following Gow's killing, there also have been renewed calls from some right-wing members of Parliament to reintroduce internment without trial and toughen legal restrictions on reporting IRA activities. But many lawmakers believe such emergency steps would only play into the hands of the IRA and give it a propaganda victory.

British officials insist that their security forces have been effective in pursuing those responsible for the bombings. "You can't look just at yesterday," a senior security source said. "Over the past 20 years, our record is fairly good. The {IRA} doesn't just fly in with a bomb. They are much more sophisticated now, and they've learned a lot from our successes. But we have the advantage of patience and time. We will find them."

Security officials say the IRA has come a long way since 1973, when a gang of bombers was captured at Heathrow Airport attempting to board a plane to Dublin immediately following an attack in London. Since IRA members bombed a military barracks in north London in August 1988, the new operatives have varied their targets, methods of attack and escape routes.

In recent months, they have bombed traditional targets such as military offices and barracks and car-bombed individual soldiers. They also have attacked highly visible institutions such as the London Stock Exchange and the conservative Carlton Club.

Most of the operatives responsible for the current wave of attacks probably were infiltrated into the British mainland over a two-year period, according to security sources. Some are hardened gunmen from Northern Ireland, but many are young, better-educated recruits from the IRA's "southern command" in the Irish republic who have no history of IRA involvement and no police record in Britain.

They are believed to be operating out of two small cells, known as "active service units." Members of each cell have no contact with or knowledge of those in the other one. Some operatives travel to the mainland from Ireland bringing orders from the IRA's high command, or weapons which they store in underground caches. That stock includes some 120 tons of rifles, machine guns, rocket launchers, ammunition and plastic explosives obtained mostly from Libya in the mid-1980s, security officials say.

In addition to the operatives, each IRA unit works closely with one or more front men -- members of the community who rent safe houses and cars and provide other logistic services for their colleagues. Both the operatives and the front men generally try to avoid contact with Britain's large Irish community, whose members are often hostile to IRA activity and which is believed to be heavily penetrated by British intelligence.

IRA officials say they conduct attacks on mainland Britain as well as in Northern Ireland because such assaults have a much higher impact on the British public.

"It is necessary, especially in a guerrilla campaign, to attack the enemy on ground which you choose," said an IRA military spokesman in a recent interview in Republican News, the weekly newspaper of Sinn Fein, the IRA's legal political wing. "To surprise the enemy, give them no rest, continually pressurize them and, of course, hit them where it hurts."

It is also easier at times to organize hits on the mainland, where people are far less security-conscious than in heavily guarded Northern Ireland, where the number of monthly attacks is far greater, but so is the number of arrests.

The Royal Ulster Constabulary announced charges Tuesday against two men arrested for last week's killing of a Roman Catholic nun and three policemen. By contrast, police have yet to charge a single person in the bombing wave on the mainland.

The few recent police successes here have been due in part to luck, and some have revealed important information about the way the IRA operates. Perhaps the most significant discovery was in December 1988 when an IRA safe house was found in south London after police came upon an IRA cell member who had shot a person attempting to break into his car.

The cell member and another IRA operative living in the house managed to evade a nationwide manhunt, but police were able to seize 11 weapons and 100 pounds of Semtex explosive. After police left, workmen at the site found a hit list of 100 names, including Gow's, on cigarette papers hidden behind a radiator and under a carpet.

The safe house discovery eventually led police to Nicholas Mullen, a London-born electronics dealer of Irish descent who recently was convicted and sentenced to 30 years for renting the apartment, buying cars, opening bank accounts and providing false birth certificates and other documents to the IRA cell members.

According to terrorist expert Wilkinson, the authorities would do better if they increased the coordination and sharing of intelligence among the eight different British security agencies that gather information on the IRA, improved the information flow from the Irish police and set out to recruit more informants by being more generous in reducing sentences for "repentant terrorists." In the long run, Wilkinson said, the 12-nation European Community should develop its own criminal justice system to deal with cross-border terrorists and drug dealers and introduce a continent-wide computerized identification system.

While insisting that intelligence coordination is already effective, the government is examining proposals for a national criminal intelligence unit to pool information among Britain's 43 local police forces. The government also has spent over $150 million in the past year to improve security at British military bases.

Meanwhile, members of Parliament today proposed a stopgap measure -- special security mirrors so that lawmakers can check under their cars each morning for bombs.