MARGARET THATCHER'S dedication to the Falklands continues to astonish. She has just made the grueling 23-hour, 8,000-mile flight to help the residents celebrate the 150th anniversary of Britain's takeover of the islands from Argentina. To be sure, any place worth sending troops to fight and die for--the British toll was 255--is worth a prime minister's visiting. But in making the visit, Mrs. Thatcher nails Britain more firmly into a position it will eventually have to abandon.

To see why, you must realize that the 23-hour, 8,000-mile flight is not merely a measure of Mrs. Thatcher's patriotism but of the true isolation of the Falklands. Latin solidarity with Argentina has meant that civil air service from the mainland has not resumed, and probably will not resume so long as the British insist that the sovereignty of the islands is not negotiable. Sovereignty was on the table for the 17 years of fruitless negotiation before the war came last April, but Mrs. Thatcher took it off as the British fleet closed. British title is now no less legally clouded than it had been for the previous 150 years. But Mrs. Thatcher stands on a position that precludes an ultimate settlement.

Meanwhile, since the Argentine grievance remains raw, the islands must be defended. A light defense will not do, given Argentina's invasion last spring and Mrs. Thatcher's embarrassment for not having been adequately prepared for it. The cost of recapturing the islands, garrisoning them for four years and making them livable is estimated at about $4.8 billion--on the order of $3 million for each of the 1,600 Falklanders-- out of a defense budget now running at $25 billion a year. Even for a country whose economy was not a disaster area, this would be an immense bill to pay.

Britain's friends owe their best judgment: the wise course is to find a way to turn sovereignty over to Argentina under conditions ensuring fair treatment of the interests of the islanders. We still don't think that's an impossible dream.