Soviet Foreign Minister Eduard Shevardnadze said today that Moscow's insistence that Britain and France freeze their nuclear forces at current levels does not affect those countries' plans to "modernize" their nuclear arsenals.

British officials, while noting that Shevardnadze's statement left numerous unanswered questions, characterized it as evidence of a "slight movement" in the Soviet position, which has been a principal sticking point at talks in Geneva between the United States and the Soviet Union on intermediate-range nuclear weapons.

Of the range of East-West arms control negotiations now under way, Shevardnadze said, "my personal view is that the most promising area" for agreement is over deployment of intermediate-range weapons in Europe.

Shevardnadze's comments came in a news conference at the Soviet Embassy at the end of a three-day official visit here that both governments have characterized as successful. Prime Minister Margaret Thatcher has accepted an invitation, brought here by Shevardnadze, to visit Soviet leader Mikhail Gorbachev in Moscow next year.

Appearing relaxed and occasionally joking during the 45-minute press session, Shevardnadze continued the upbeat and optimistic attitude toward East-West relations, particularly in the area of arms control, adopted by several Soviet officials in recent weeks.

On the question of a summit between President Reagan and Gorbachev, Shevardnadze said that the Soviet Union was "working hard to prepare" for it. He said that "substantial preparations are under way" in Moscow in "10 to 12 areas" to be reviewed at a preliminary meeting between himself and Secretary of State George P. Shultz and then to be discussed at a summit.

But he repeated Soviet insistence that dates for neither a Shevardnadze-Shultz meeting, nor a Reagan-Gorbachev summit, can be set until Washington responds to existing Soviet proposals for strategic weapons cuts.

"The ball is now in the U.S. court," Shevardnadze said. "A great deal will hinge on the answers the United States gives us."

Asked if Moscow was growing frustrated with the lack of a U.S. response, he counseled patience. "The United States is in the process of preparing a reply," he said. "Wait and see. I said the ball is in the U.S. court, but I didn't say we have scored a goal against the Americans."

Among the other points made during the session, Shevardnadze:

Made Moscow's first public reference to U.S.-Soviet talks on nuclear testing that are due to begin next week in Geneva. "We have a fundamental agreement from the United States to resume negotiations . . . on banning nuclear tests," he said.

Although both sides have agreed that the talks will begin "without preconditions," the Soviets want to discuss a comprehensive ban on all nuclear tests, while the Reagan administration has said continued testing is necessary until agreements are reached to eliminate nuclear weapons.

In Washington, the White House issued a statement Wednesday saying that "the United States and the Soviet Union have agreed to have experts meet shortly to discuss issues related to nuclear testing," Washington Post staff writer Lou Cannon reported.

Said that Moscow hopes to "convince the United States that we have very good arguments" that it is not violating the Antiballistic Missile Treaty and the Salt II agreement on strategic weapons limitation during talks on the issue that Washington recently agreed to in response to a Soviet request.

Said that "on the question of space weapons," including the Reagan administration's Strategic Defense Initiative, which Moscow opposes, "there is a possibility of finding more mutually acceptable approaches."

Referred to chemical weapons negotiations, on which Britain yesterday proposed a new verification formula at the Geneva Conference on Disarmament, as an area where Britain and the Soviet Union could undertake "joint work."

But Shevardnadze expressed the most enthusiasm over the possibility of East-West agreement on limiting intermediate-range weapons in Europe. The Soviets have proposed reduced deployments of their SS20 missiles, to be matched by reduced numbers of U.S. Pershing II and cruise missiles in Western Europe.

Although the Soviets long have insisted that British and French nuclear weapons be included in any accounting of western deployments in Europe, they have said recently that those non-U.S. arsenals should be negotiated separately with London and Paris.

The two remaining Soviet conditions on exempting the British and French weapons from talks with Washington on intermediate-range weapons, however, have been that neither country expand its nuclear arsenal in the meantime and that the United States agree not to transfer "nuclear weapons to other countries."

That position has been interpreted to rule out Soviet acceptance of British and French plans to modernize their nuclear arsenals, both because the modernization plans allow for substantial expansion in the number of each country's warheads, and, in the case of Britain, because it is modernizing its forces with a new nuclear weapons-equipped Trident submarine system purchased from the United States.

Both British and American officials noted that Shevardnadze's comments on the subject today left unclear whether acceptance of British nuclear "modernization" meant acceptance of a Trident system.