BRITISH PRIME MINISTER Margaret Thatcher, by restructuring the Royal Navy, has kicked a big hole in the North Atlantic Treaty Organization's plan for saving Europe from the Soviets in any next war.

That's why the Royal Navy's success in the Falkland Islands is of little cheer to the Big Picture men planning NATO war strategy, particularly for American admirals trying to figure out how to keep Europe supplied if the shooting starts.

Thatcher enlarged their problem by scrapping one major aircraft carrier and many surface warships so Britain could afford to buy the Rolls Royce of U.S. missile submarines, the giant Trident. U. S. military planners would much rather have lesser British warships prepared to combat Soviet submarines in Europe's home waters. The reason: Soviet submarines, like the German U-boats of World War II, are considered the chief threat to the America- to-Europe supply line if war should come.

"The Soviets' surface navy is not the principal problem," Adm. Thomas B. Hayward, chief of naval operations, told Congress shortly before his retirement this year. "It is their submarine navy and the air- launched missiles that create the principal threat .. ."

54 Confronted with a latter-day version of the German U-boats, the United States and its NATO partners for the last two decades have been designing and testing a giant antisubmarine net as part of their barrier strategy. The idea is to overcome the Soviets' advantage in numbers by keeping many of their submarines from ever reaching the open ocean, where they could sink allied ships carrying troops, ammunition, tanks and fuel from the United States to Europe. Geography made the barrier strategy an obvious choice.

Soviet submarines and surface ships have to sail through narrow chokepoints to get out of Baltic Sea ports to the Atlantic. Soviet ships sailing from Black Sea ports to the Mediterranean similarly must get through the Dardenelles chokepoint.

The same chokepoint vulnerability confronts much ofthe Soviet Pacific fleet because of the narrow straits it must clear to go from Vladivostok on the Sea of Japan to the North Pacific Ocean. Japanese ports and air bases are so vital to the barrier strategy that U. S. military protests against low Japanese defense budgets are bound to remain muted for fear of losing those antisub launching pads some day.

While it is comparatively easy to keep the Baltic, Black Sea and Sea of Japan gateways guarded, geography has been less kind to NATO in the far north in the waters between the Arctic Ocean and the Northern Atlantic. The Soviets' powerful Northern Fleet can sail from a port like Murmansk directly out into the open waters of the Barents Sea.

So the NATO war plan calls for falling back to the narrow stretches of water between Greenland, Iceland, Norway and Britain to make the big stand against Soviet submarines of the Northern Fleet. Soviet subs would have to get through the gaps to sink ships ferrying troops and supplies between America and Europe. NATO planners figure their ships must make 3,000 trips from the United States to Europe in the first six months of a war if they are to deliver the required 1.5 million troops and 28 million tons of weapons, fuel and other supplies to sustain forces fighting in Europe.

Soviet missile subs would also be in a better position to hit U. S. bomber bases and missile fields if they got through those gaps. Small wonder, then, that the planners have been concentrating for the last two decades on ways to stretch an antisub net from Greenland to Iceland to Britain and Norway to keep the Soviets out of the North Atlantic where they could strangle the NATO alliance.

The net has been designed, deployed and exercised as the alliance has waged make-believe war against Soviet submarines. The net consists of mines, which, when activated by the distinctive characteristics of Soviet subs, would jump off the ocean bottom and slam into them; American attack subs lying in ambush along the Soviets north-to- south route; underwater microphones for tracking subs; land- and carrier-based antisubmarine aircraft; AWACS (airborne warning and control systems) flying out of Iceland to coordinate sub killing forces.

Any Warsaw Pact takeover of air bases in Norway or Iceland would rip away key anchor points of the anti- sub net. So war plans call for rushing aircraft carriers to help defend those countries. Also, U. S. Marines in the past few years have been practicing ways to combat Warsaw Pact forces that might invade Norway.

But despite all this effort and expense, the net across the upper end of the North Atlantic is not leakproof. Some Soviets subs would get through to threaten to the U. S.-to-Europe lifeline after a war started. Others almost certainly would have passed through the net before the shooting began.

So NATO navies would have to hunt down and sink these "leakers" -- to use a missile term -- and escort cargo ships in the old convoy style of World War II. And here is where Prime Minister Thatcher has created the gap which keeps NATO planners from cheering the navy that won the Falklands battle.

"I am dissappointed by the British action to reduce their destroyer and frigate force by about 30 percent, from 59 to 42 ships," said Adm. Harry D. Train II, who until last month headed the Atlantic Command charged with keeping Europe supplied in a war. "Prior to this decision, the United Kingdom provided about 70 percent of the surface units available in the eastern Atlantic.

"Despite our forward defense strategy," the submarines that pass through NATO barriers before a war starts, or penetrate it afterward, "will make direct defense of shipping a necessity, especially in the waters near Europe," Train noted in his farewell statement to Congress.

"All our exercise and war game experience tells us that more frigates and antisubmarine helicopters are needed. Even in units of one or two they are effective in the narrow seas near Europe."

The British Defense Ministry says that the smaller ships it will build to replace the bigger ones will pack plenty of punch to help protect the lifeline. But critics here and abroad contend the Falklands demonstrated that Britain needs the bigger shjips Mrs. Thatcher's government is forsaking. They underscore that assertion by pointing to the fact that the flagship for the Royal Navy's Falklands operations, the Invincible, is slated to be sold to Australia. British Defense Minister John Nott offered no hope to the Adm. Trains of the alliance in his assessment on the Falklands, given June 22. He said the Thatcher government plans to go full speed ahead with the $13.5-billion Trident deal whether admirals on both saides of the Atlantic like it or not.