Astride the barren, muddy flatland of this 40-year-old air base, a massive load of concrete and steel is being molded into six shelters for the nuclear-armed cruise missiles that will begin arriving later this year.

"The client takes possession of the first shelter in June," said a British government engineer, archly referring to the U.S. Air Force's 501st Tactical Missile Wing that will be manning the weapons.

The occasion for his comment was the visit here Thursday of Defense Minister Michael Heseltine, who donned a hard hat and camouflaged field jacket over his pin-stripe suit to trek through the site in a public display of Britain's commitment to the missiles. With reporters and photographers in tow, Heseltine left no doubt that the Thatcher government is, as he put it, "resolute" in its belief that deployment should proceed unless the Soviets agree to dismantle their medium-range SS20s targeted on Western Europe.

Attention to the political debate over U.S. negotiating positions and popular protests against the new generation of nuclear weapons has tended to overshadow the fact that in Britain, at least, preparations to accept the missiles are proceeding apace. For all the emotional impact of the Greenham Common women's peace camp pitched just beyond the base's security fence, the deployment they so bitterly oppose will be an established fact in a matter of months.

"For many people Greenham Common has become a symbol," Heseltine declared in recognition of the women's celebrity. "For me too it is a symbol, a symbol of NATO's determination to ensure the continuing success of its policy of deterrence."

Because opinion polls show a majority of the British public opposed to the siting of the subsonic mobile cruise missiles in Britain, Prime Minister Margaret Thatcher has ordered Heseltine, who became defense minister in January, to mount an all-out effort to gain support for the weapons buildup. Heseltine, nicknamed "Tarzan" by the press because of his distinctive mane of blond hair, is one of the Conservative Party's leading orators and a formidable advocate for whatever cause he chooses to support.

His doggedness is characteristic of Thatcher's own style. Heseltine has quickly emerged as the most outspoken defender of the missile program in a Europe that, on the whole, seems to prefer that the Reagan administration spare it the headaches of pressing ahead by making a deal with the Soviets.

Barely a week after moving from the Department of the Environment to Defense, Heseltine was appearing on television arguing on behalf of the cruise missiles. He set up a special Secretariat 19 within his department to coordinate government efforts to counter the antinuclear movement, and he meets regularly with an interdepartmental public relations group to determine further strategy.

In speeches on the stump and in Parliament, meetings with reporters and articles in newspapers, Heseltine has refined his case. He uses the now standard NATO formulation endorsing President Reagan's zero option as an ultimate objective while expressing willingness to consider any other proposals, as he put it this week, "provided there is a balance and no phony counting."

"We have made it clear that the end of 1983 is not the deadline for negotiations," his text for the Greenham Common visit continued. "We are quite prepared at a later stage to halt or reverse deployment if progress in the negotiations with Moscow warrants it."

Notwithstanding these expressions of flexibility, Heseltine is adamant that no amount of protests by what he contends is an unrepresentative fraction of the British public will deter the government from the deployment program.

A substantial majority of the population consistently has supported the government's defense policies, Heseltine contends, and skepticism about cruise missiles can be overcome by explaining why they are necessary.

"We were deeply involved in the Falklands for the better part of the past year," the minister said in an interview last month. "If we'd spoken out on cruise, we would have been ignored. Now our views are becoming better known."

Heseltine and other government spokesmen even see progress for their position in the widespread calls from across the political spectrum for a "dual key" on the missiles, giving Britain a physical means of restraining the United States from firing the warheads. He asserts that long-standing agreements with the United States already ensure that use of the weapons is subject to a "joint decision" process between Washington and London. By discussing those procedures, he says, critics are acknowledging that the missiles are necessary.

In any event, Heseltine believes that the dual key controversy is primarily a ploy intended to find grounds for putting pressure on the government.

"If they got dual key," he said, "they'd shift the argument to something else."

The effectiveness of Heseltine's efforts to counter the antinuclear movement will get a major test over the upcoming four-day Easter weekend, a national holiday. The Campaign for Nuclear Disarmament, continuing a practice that dates from the early 1960s, is planning marches and demonstrations around the country, joined by supporters of the Greenham Common peace camp.

One of the biggest demonstrations is to be a 14-mile human chain of about 40,000 people stretching from here to the Burghfield Royal Ordnance factory where nuclear weapons are made.

Asked to comment on the Easter plans, Heseltine said, "they are masterminded by a small group of politically motivated people using high-profile techniques" to attract publicity. "But that," he added, "doesn't make their case any stronger."

In recent weeks, the Greenham Common women have been subjected to various legal proceedings aimed at evicting them from the site near the base's main entrance where the camp has been for 18 months. The government insists that it takes no part in such actions, which are the responsibility of local authorities. But the effect of the court actions has been to heighten the sense of confrontation between a government intent on proceeding with a nuclear buildup and a core of dedicated protesters.

After inspecting the construction of the concrete shelters--known as garages--where the missiles are to be housed, Heseltine told reporters he was not yet satisfied with security around the base's nine-mile perimeter. Over Christmas dozens of women from the camp scaled the 10-foot chain-link fence and danced atop a building site for 40 minutes before they were removed by police.

The first 16 of an eventual total of 96 missiles are to be flown in from the United States as soon as their garage is ready in June. Once made operational in December, they will be mounted on mobile "transporter erector launchers" designed to be dispersed into the countryside in the event of war. To protect the shelters from the most determined intruders, a system of alarms and specially designed barbed wire obstacles is being installed.

When the project is completed, 2,200 U.S. Air Force personnel and their families will be living in and around the base.

Unless the peace camp is disbanded, many of the Americans will be passing it each day as they enter and leave.

A hint of the problems ahead on this score came this week as the buses carrying reporters were spirited through a back entrance. Five women suddenly appeared and lay down in the road, snarling traffic until police arrived.

As for Heseltine, he left Greenham Common, as he had come, by helicopter