British-Iranian ties strained over actions against diplomats. Britain weighs response to Iran's detention of official in Tehran

London and Tehran are at loggerheads over Iran's detention and beating of a British diplomat. As the dispute intensifies, a complete diplomatic break between them seems possible. Edward Chaplin is No. 2 in Britain's diplomatic mission in Tehran. Last Thursday, Iranian Revolutionary Guards grabbed Mr. Chaplin from his car there, held him for 24 hours, then ``temporarily'' released him. The move appears to have been triggered by the May 9 arrest on shoplifting charges of an Iranian consular official based in Manchester, England.

British Embassy staff said Chaplin had been beaten while in custody. The Foreign Office in London described his treatment as a ``preposterous and intolerable breach'' of diplomatic rules under the Vienna Convention.

Chaplin's seizure swiftly escalated into a full-scale incident as the British foreign secretary, Sir Geoffrey Howe, broke off election campaigning yesterday and hurried to London to deal with the problem. The Iranians, meanwhile, continued to complain bitterly about the arrest of their official in Manchester.

The diplomatic standoff was made to appear sharper by the fact that Britain is in the midst of a general election campaign. The Thatcher government decided that it must adopt a tough posture or face criticism from its political opponents.

Still, the issues involved are serious, and Sir Geoffrey has taken a firm stand. He has apparently made a number of decisions on steps Britain will take, but at press time, the steps had not been announced.

Economic sanctions against Iran, which last year imported 400 million ($651 million) of British goods, were said to be under active consideration by the Thatcher government. Since the US Embassy crisis in 1980 Britain has not had full diplomatic relations with Iran. A small British diplomatic unit operates from the Swedish Embassy in Tehran.

Although Chaplin's detention was apparently in retaliation for the arrest in Manchester, the Foreign Office said it had no intention of dropping the charges against the Iranian consular officer. He is also accused of reckless driving and assaulting a police officer.

Iran has said it may try Chaplin on ``serious charges'' if he tries to leave Iran. This threat has angered the Foreign Office. British officials summoned the Iranian charg'e d'affaires and reminded him that Chaplin enjoys full diplomatic immunity whereas the Iranian consular official has only limited immunity.

Sir Geoffrey is said to be worried that the Iranians were behaving in a manner similar to the Revolutionary Guards who sparked the US Embassy crisis eight years ago. British officials said Chaplin behaved correctly and the Revolutionary Guards had flouted the law.