For Americans, Ronald Reagan's speech on South Africa was merely shameful, another instance where the president will have to be rescued from his folly by people who have a firmer grasp on reality.

For the British, it was more distressing. While Americans can expect that the likes of sober-sided Richard G. Lugar, chairman of the Senate Foreign Relations Committee, will produce a law that repudiates the president's specious defense of the fascists in Pretoria, the British have little to look forward to.

The British are facing the elimination of the last shreds of imperial pride. The Commonwealth, a civilized and benign assemblage of ex-colonial nations that often say and do helpful things for the world, is being splintered over sanctions against South Africa.

Prime Minister Margaret Thatcher and Reagan stand alone together, like Greer Garson and Walter Pidgeon in a World War II weeper. Thatcher, at least, is consistent. She has always opposed sanctions. Reagan, on the other hand, slaps them on left-wing offenders. They are "punitive" when contemplated against South Africa. Against Cuba, Libya and Nicaragua, they are simply strong economic measures taken against Marxists.

Thatcher's intransigence, it is said, has caused Her Majesty Queen Elizabeth II to take the unprecedented, and unconstitutional, step of expressing her displeasure at the prospect of presiding over the disintegration of the empire. Quotes and facts are elusive, but it is now part of the dialogue that the queen has told Thatcher she does not wish to be on the spot when the sun finally sets on the remnants.

The distressful prospect of divorce from the glory of empire has been looming during the delectable pomp of the latest royal wedding. The British genius for organization, for felicitous, flawlessly executed detail was in full view as Sarah Ferguson and Prince Andrew, royalty's new fun couple, took their vows in Westminster Abbey.

The queen wore an expression of unrelenting glumness. Perhaps she was belatedly aware of having been done in once again by her milliner. Her Majesty, admirable though she is, has in the matter of fashion an almost actionably flawed judgment. Her ensemble was a particularly mournful shade of blue, topped off with a disastrous hat, a wide-crowned affair with a gratuitous binding of material across the brow that made her look as if her head were bandaged.

Or possibly, she was infected with the tension in her immediate vicinity, the palpable parental anxiety of the prince and princess of Wales, whose son William was making his public debut. British courage is never more evident than in their practice of making small children members of the wedding. The valor of Agincourt was needed to give William, an obstreperous 4-year-old, a role as a page and to launch him down the aisle of the Abbey within kicking distance of the bride's 17-foot train.

Thatcher was also in the Abbey, looking pensive. She is said to be entirely comfortable in the storm she has created. She shares fully in Reagan's belief that the government of P.W. Botha is making progress in ending apartheid. Like him, she thinks that withholding the judgment implicit in sanctions is an act of high morality, based on an aching concern that South African blacks, rendered jobless, would suffer more than they have under the whips and chains of their oppressors.

Bishop Desmond Tutu made an appropriate response to the president's silly restatement of "constructive engagement." It was "nauseating," he said. Capitol Hill said "amen" in less vehement terms.

For Reagan, the South African crisis is the Philippines revisited. Again, he gave us the egregious "evenhandedness" of his utterance about the Philippine election, in which he said both sides cheated. In South Africa, he said, blacks must share the blame for the violence that is engulfing their country. They should be patient. At least those who are beating, jailing and torturing them are not Marxists.

Thatcher recently put her objection to a stand against the fascists in Pretoria on humanitarian grounds.

But in the end she gave the argument that always carries with her and Reagan, the red menace. Strategic materials, she said, are available from South Africa or the Soviet Union. "To me, it is utterly absurd that people should be prepared to put increasing power into the hands of the Soviet Union on the grounds they disapprove of apartheid in South Africa," she said.

There you have it, an expression from one of the two last leaders in the world who will risk anything in the way of opprobrium -- and disintegration -- to close a "window" for communism in a country where fascism has long since come through the door.