LONDON, AUG. 19 -- Britain has undertaken a forceful diplomatic effort to try to broaden international political cooperation and military participation in the Persian Gulf, expressing concern about U.S. isolation, its own newly exposed position and the general escalation of tension there.

Over the past 10 days, Prime Minister Margaret Thatcher has dispatched messages and emissaries to several Western European countries and Japan, asking them to join or otherwise aid British mine-sweeping efforts in the gulf.

At the same time, the government here has warned the gulf states that British mine sweepers will not be available to clear the waters of those countries that do not make their ports and other facilities available to the British fleet.

Thatcher also has exchanged private communications with Soviet leader Mikhail Gorbachev in the belief, unshared by many U.S. policy makers, that the Russians must be directly involved in any international solution to the immediate gulf navigation problem, as well as to the long-term Iran-Iraq war.

Now that Britain has been more directly "drawn in" to the situation by its decision last week to send mine sweepers, "we wish to give a lead," a senior government official here said. He emphasized that Britain was supportive of U.S. policy, but worried that the situation in the gulf could become a dangerous U.S.-Iranian confrontation, or a source of friction between the superpowers.

Reacting to what one western diplomat here estimated will soon be a U.S. military presence of up to 25,000 personnel in the gulf, the official said it was "not for us to dictate what scale the United States believes is necessary to meet its commitment."

But, he said, it now "needs the whole world protesting," rather than just the Americans, to make an impression on Tehran. "The Iranians have no interest in a U.S. attack," he said. "We hope that if we can get on top of the minelaying, it can be turned into a war of words."

The gulf crisis also is seen here as an opportunity for Thatcher, recently elected to a third successive term, to exercise her "position of influence" as the senior statesman of the western alliance.

The diplomatic effort has met with mixed response. Thatcher's requests for mine-sweeping assistance from West Germany, Belgium, the Netherlands and Italy came after all had turned down U.S. appeals. In letters to the head of each government, Thatcher is believed to have pointed out that all of those countries receive more of their imported oil from the gulf than does Britain. So far, France has been the only other West European nation to announce it is sending mine sweepers to the area.

That point was emphasized last weekend by Foreign Office Minister David Mellor, who said in a radio interview here that countries that have advocated a United Nations peace-keeping force, rather than making their own efforts to lessen gulf tension, were indulging in "escapism," since there was no indication that the United Nations is ready to organize such a force.

Mellor's comments brought swift, critical responses from Italy and the Netherlands, both of which have supported calls for a U.N. force. In The Hague yesterday, the British ambassador was summoned to explain Mellor's comments to the Foreign Ministry.

The Netherlands, as current chairman of the West European Union, the long-dormant, seven-member European defense organization currently in the process of revival, has called for an emergency meeting in The Hague on Thursday to talk about coordinated efforts in the gulf.

Britain's appeals for shore facilities in Oman, Bahrain, Qatar and the United Arab Emirates have left those countries in a dilemma as to how to take advantage of British and French mine sweepers being sent to the area without provoking Iranian retaliation. The most positive response is likely to come from Oman, which has particularly close military training and supply ties with London.

In the meantime, Britain has sought to convince the Soviet Union that its approval last month of a U.N. Security Council resolution calling for an Iran-Iraq cease-fire also implied approval of a possible arms embargo against whichever party to the war refuses to comply. The Soviets, along with China, have resisted talk of an embargo, with Moscow arguing that mediation efforts should be given more time.

Thatcher has used her personal relationship with Gorbachev, solidified during her Moscow visit last spring, to appeal for a coordinated U.N. embargo. On Monday, Soviet Ambassador to Britain Leonid Zamyatin delivered Gorbachev's response to an earlier Thatcher letter on the matter. While officials here said Moscow and London still differ on "one or two points," they said Britain would continue to try to closely involve the Soviets in international efforts.

Britain began to press its high-profile diplomatic effort in conjunction with Thatcher's decision, announced Aug. 11, to send four of its sophisticated, Hunter-class mine sweepers to the gulf. Although the decision followed her rejection, 10 days earlier, of a U.S. request for mine sweepers, officials here vehemently denied they had been subjected to U.S. pressure.

Thatcher, who supported the Reagan administration's decision to reflag Kuwaiti oil tankers and provide U.S. Navy escort for them in the gulf, was said by officials here to have been personally distressed by her inability to respond to the American request. But, they said, both her Defense and Foreign ministries had strongly advised against it as likely to lead to an escalation of tensions in the gulf.

The reversal, when it came, was tied to "new circumstances" in the Persian Gulf area, principally the discovery of apparently Iranian-laid mines in the hitherto safe waters of the Gulf of Oman, outside the Strait of Hormuz. This is the area of operation of Britain's Armilla patrol, the two to three warships that for the past eight years have been escorting British flag vessels through the strait and as far into the gulf as Bahrain.

According to official sources here, the new mines resulted in a changed military assessment by the Defense Ministry, and a specific request for mine sweepers from First Sea Lord Sir William Staveley to protect the Armilla patrol.

In addition to the military assessment, other Cabinet objections were eased by new political calculations. The risk of being seen as kowtowing to the Americans by protecting their vessels in the gulf became outweighed by the risk of a British vessel being blown up because of government refusal to protect Britain's own ships.