London — Major policy differences are straining relations between the United States and its ally, Britain. Britain is concerned not just by the buildup of American naval and air forces in the eastern Mediterranean and the prospect of retaliatory action by the United States for the massacre of its marines in Beirut.
Beyond that, Prime Minister Margaret Thatcher is still smarting over the American intervention in Grenada and the lack of adequate consultations beforehand.
Her continuing displeasure emerged this week when she met with Undersecretary of State Kenneth W. Dam at her country residence, Chequers. Mrs. Thatcher still has not endorsed the US action.
The prime minister's office commented only briefly on Mrs. Thatcher's 90 -minute discussions with Mr. Dam, who is visiting Western Europe to ''mend fences'' after the strain put on the Western alliance by the Grenada incursion.
But according to US officials in London, Mrs. Thatcher bluntly told Mr. Dam that by invading Grenada, the US had given the Soviet Union a powerful propaganda weapon. The feeling at 10 Downing Street is that it has heightened anti-American sentiment and added to the furor over the installation of US cruise missiles in Britain.
There are signs the circumstances surrounding the US action may have strengthened the hand of British antinuclear campaigners, who maintain that, in the event of war, the US might act without taking full account of the opinions of its allies.
Mrs. Thatcher elaborated on this theme in a frank interview Monday in the conservative newspaper the Daily Mail. She said the clear message of the Grenada incident was that Britain and the US would have to open up a clearer line of communication.
During talks with the US envoy, Mrs. Thatcher also voiced strong concern over the possible resumption of US arms sales to Argentina, following the elections there. Nevertheless, according to American sources here, she failed to secure any firm assurance on the matter.
Dam said Washington had taken no final decision, but the question would have to be considered if Argentina's human rights situation improved under the new civilian government of President Raul Alfonsin.
Britain insists normal relations with Argentina cannot be restored until that country concedes defeat and renounces its claim to sovereignty in the Falkland Islands.