London — As Britain prepared to enforce its ''maritime exclusion zone'' around the Falkland Islands April 12, there were signs that a naval confrontation might be delayed to give diplomacy more time to work.
Except for one destroyer and one frigate, Argentine naval ships had returned to port. ''I expect no targets,'' British Foreign Secretary Francis Pym said April 11. It was up to Argentina, he said, to keep its ships out of the 200-mile radius around the Falklands.
Chief among the pressures on London to exercise restraint was the mediation mission of US Secretary of State Alexander M. Haig Jr. Mr. Haig was due back in London from Buenos Aires shortly after the blockade was to begin and was said to be carrying new ideas on possible shared sovereignty to present to Prime Minister Margaret Thatcher and Mr. Pym.
A number of commentators here were saying on the eve of the blockade that the British government might lose public support at home and abroad if it opened fire soon.
''The biggest tragedy of all would be if Britain struck when a peaceful settlement was still possible,'' the Financial Times commented.
The Sunday Times quoted a 19th-century French strategist: ''There is nothing so hard in war as the heroic feat of holding back.''
An early British salvo would not only boost the standing of Argentine leader Leopoldo Galtieri at home, but also could persuade him to seek open help from the Soviet Union.
Soviet concern about the British task force and the blockade appears to be growing. A sophisticated electronic surveillance ship is shadowing the liner Canberra, which is carrying 2,000 British troops to the South Atlantic. The Soviet ship took particular interest in an exercise in which the Canberra was refuelled at sea. British officials are concerned that the Soviets might be sending intelligence findings directly to Buenos Aires.
It is also reported here that the first ships in the exclusion zone to be sighted by British nuclear submarines Superb and Spartan might be Polish and not Argentine.
Three Polish freighters have been anchored outside the harbor of Port Stanley , capital of the Falklands, for the past week.
Britain has said its blockade is against Argentine ships and has not mentioned other countries' ships.
Asked by the Sunday Times what advice he would give to the Polish ships, a Ministry of Defense official in London replied, ''Take care.''
Although the Argentine junta is an open opponent of Marxism, Argentina is selling large quantities of grain to the Soviet Union. Moscow abstained in the 10-to-1 Security Council vote in New York calling for an immediate Argentine withdrawal, a cessation of hostilities, and peace talks. But the Soviets have denounced the British task force in public, and may try to take advantage of Argentina's growing isolation.
The isolation intensified April 10 when the European Community in Brussels unanimously decided to ban all Argentine imports. Britain is delighted at the unanimity of the decision, which hits West Germany and Italy harder than London. Germany's trade with Argentina is three times that of Britain, and Italy uses Argentine leather in its shoes.
Yet the decision also provides an opportunity for Moscow to become a closer trading partner of Argentina.
There is no doubt here that British submarines could sink any ship they are likely to find within the 200-mile zone.
Propelled by silent Rolls-Royce nuclear reactors, the submarines can travel at 30 knots, detect ships 40 miles away, and fire Tigerfish torpedoes with 600 pounds of explosives at a range of over 30 miles with extreme accuracy.
But with Mr. Haig returning here, the main stress is on diplomacy. Mr. Haig reportedly carried Argentine suggestions about shared sovereignty of the islands: The islanders would hold duel Argentine and British citizenship and the islands would be jointly administered.
The islanders have rejected this plan in the past. They have also objected to a ''lease-back'' arrangement whereby Argentina would take sovereignty and immediately lease the islands back to Britain for an extended period.
The governor of the Falklands, Rex Hunt, said in London April 11 he was convinced that the Falkland Islanders wanted to remain British at all costs. He said a letter recently delivered here from the islands and signed by several civil servants, urging the British government to be cautious and asking for help in emigrating, did not represent the majority island opinion.
Meanwhile a new opinion poll conducted for the British commercial television network ITV indicated that 78 percent of 1,000 surveyed on April 8 supported British policy but that almost 60 percent of the same people wanted no Falkland or British lives to be lost.