With a British invasion of the Falkland Islands appearing imminent, Britain has now mobilized about 22,000 men in a task force of more than 70 ships strung out over the Atlantic Ocean at an estimated cost of more than half a billion dollars.

That means Britain is using 12 men and spending at least $275,000 for each of the 1,800 islanders in its undeclared war with Argentina to regain the islands seized more than six weeks ago.

All the indications today were that Britain was preparing to assault the islands soon unless a diplomatic settlement can be reached this week through the mediation of U.N. Secretary General Javier Perez de Cuellar.

A British invasion would change the nature of the war, testing the morale of the estimated 9,000 Argentine troops on the islands and at the same time making British forces more vulnerable to air attacks--since they would be less mobile than they are on the ships.

For the most part until now, Argentina, having control of the islands, has let Britain set the pace of the fighting. A British invasion, however, is likely to cause far more response because Argentine troops would be under direct threat.

Military analysts say there are numerous potential invasion sites, given the miles of unpatrolled coastline on the two main islands--whose total area is slightly smaller than Connecticut.

West Falkland provides a safer invasion route because there are believed to be only about 1,500 Argentine defenders on that island. It is within closer range of Argentina's land-based aircraft, however, and it is a long way from the eventual British target, the capital of Stanley on East Falkland. But a frontal assault on Stanley, where most of the Argentine force is based, could produce heavy casualties and be politically unacceptable in Britain unless a quick victory can be achieved.

The decision on where to attack could well depend upon the intelligence Rear Adm. Sandy Woodward, commander of the task force, has on the morale of the Argentine conscript Army after almost three weeks of intermittent bombardment and a blockade. There is widespread belief here that Woodward has landed special forces on the islands to carry out sabotage activities and test the will of the Argentines to fight Britain's better-trained, volunteer forces.

To recapture the islands, Britain has mounted its largest naval operation since World War II. There are at least 28 warships and auxiliaries in the task force and about 50 merchant ships, ranging in size from the 686-ton tugs Yorkshireman and Irishman to the 67,140-ton Queen Elizabeth 2, the world's largest luxury liner now converted into a troop ship carrying more than 3,000 soldiers.

The QE2, which left Britain six days ago, is still in the North Atlantic but there are about 4,500 royal marines, paratroops and commandos on task force ships in the immediate war zone.

The major warships in the fleet are two aircraft carriers, seven destroyers, five frigates and seven amphibious warfare craft, including two assault ships. There are also believed to be three submarines on patrol, but the Defense Ministry will not talk about that.

Air support is provided by an estimated 37 Harrier vertical-takeoff jets and about 35 helicopters, mainly used for transporting troops.

The Argentine Navy, now including an aircraft carrier, about 10 destroyers and three submarines, has hardly been heard from. The British task force has sunk the cruiser General Belgrano and badly damaged a surfaced submarine, disabling it. The Royal Navy, however, reportedly has not been able to locate Argentina's three other submarines.

The Argentines have great numerical superiority in the air, with more than 100 jet fighter-bombers. In their biggest success, a sea-skimming missile fired from a French-built fighter virtually destroyed the British destroyer Sheffield.

That incident brought home the fact that even though Argentina's Navy is heavily dependent on World War II ships, the war is testing a new generation of weapons.

The Argentine pilot who fired the French-made Exocet missile that sank the Sheffield, from a distance of perhaps 40 miles, doubtless saw the ship only as a blip on a radar screen, and probably did not know of his success.

The first combat firing of the Exocet has shaken some theories about nations like the United States maintaining large, costly but vulnerable surface fleets.

The British Sea Wolf missile is designed to combat the Exocet, but it has yet to be used in that capacity. It did have its first successful combat firing last week, however, downing two U.S.-built Argentine Skyhawk fighter-bombers. The Sea Wolf must be carefully controlled since it is designed to fire at any incoming missile or aircraft, enemy or friendly.

A third new weapon also had a successful debut. A British submarine fired two Tigerfish 1.5-ton wire-guided torpedoes to sink the Belgrano, a U.S. Navy cruiser in World War II.

Ironically, the British are having to contest some of their own weapons since the Argentine military is armed with British-built destroyers, missiles, helicopters and bombers. Past British arms sales to the military government have been criticized in Parliament since the Falklands invasion but the United States and France have also sold Argentina weapons.