London — It is hard to imagine a rectangle of the earth's surface more remote from the average Briton than El Salvador. Even Foreign Office officials candidly admit that British interests in the Central American republic are "minimal."
Yet, rather to Britain's surprise El Salvador played a considerable role in the discussions being held in Washington between Prime Minister Margaret Thatcher and President Reagan Feb. 26 to 28.
And recent US efforts here in London to sway British opinion against communist- bloc military aid to the opposition democratic front in El Salvador ran into difficulties -- not the least of which was the engagement of Prince Charles to Lady Diana Spencer, which all but obliterated newspaper and TV coverage of those efforts.
Mrs. Thatcher's government was all set to talk to Mr. Reagan about East-West relations, the new Soviet offer of a summit meeting with Mr. Reagan, nuclear mis siles in Europe, Poland, Afghanistan -- and economic matters. Mrs. Thatcher's efforts to slash public spending and lower taxes are in deep trouble here. But they are similar to Mr. Reagan's own new proposals to Congress.
Then came a series of emissaries from Washington asking for a show of public support from London, Bonn, and Paris for the Reagan policy of upholding the El Salvadoran government of Jose Napoleon Duarte.
European capitals found they were being presented with a mass of documents which the Americans cited as clear proof that the Soviet Union, Cuba, and their communist allies were sending large quantities of military supplies to the leftist guerrillas thus trying, in the US view, to turn El Salvador into another communist bridgehead close to US shores.
Here in London, Mrs. Thatcher is ideologically inclined to sympathize with Mr. Reagan's concern that the communist bloc is threatening to expand its presence in Central America.
But the situation is more complicated than that.
Like other Europeans, Mrs. Thatcher is concerned primarily with other matters. El Salvador is of real concern only to the extent that Mr. Reagan demands a show of support. US envoys have made it clear in London that Washington would welcome condemnation by Mrs. Thatcher of "outside interference" (meaning Cuba and ultimately, the Soviets) in El Salvador.
Following up the visit of US Ambassador Lawrence Eagleburger, Deputy Assistant Secretary of State Herman Cohen went to the Foreign Office Feb. 24, to hold a press briefing, release the documents publicly, and tape interviews with both British Broadcasting Corporation and commercial television for showing that same night.
He also visited the House of Commons to talk to an all-party group of members of Parliament on Latin America.
American sources said bravely that at least part of his message penetrated to British policymakers. They were particularly concerned to get across the point that, in the US view, President Duarte retained considerable democratic support among younger Army officers and civilians, and was carrying out a constructive program of land reform and other policies against great odds.
But the Foreign Office would do no more than agree to study his submissions. News of Prince Charles' engagement took up acres of news space in the papers and hours of coverage on television and radio. Only one newspaper (The Times) carried a report on the Cohen press briefing, and even that was brief.
British sources said it would have been better for the US to have released the documentation of arms supplies well before it did so Feb. 23. But US sources, while agreeing in hindsight, said the evidence had to go presented to European governments and released in Washington first.
"If Mr. Reagan makes support for his position a touchstone of our relations, then we'll have to consider it carefully," one British official said privately.