President Reagan yesterday defended his proposed "Star Wars" antimissile defense plan and expressed confidence that it will win the support of British Prime Minister Margaret Thatcher.

The two leaders are to confer today at Camp David. Thatcher has expressed reservations about the controversial plan on grounds that it could be an obstacle to an arms control agreement with the Soviet Union.

Asked about the concerns of Thatcher and French President Francois Mitterrand, Reagan said: "Well, I'll get them to understand it . . . . Today the only defensive weapon we have is to threaten that if they kill millions of our people, we'll kill millions of theirs. I don't think there's any morality in that at all, and we're trying to look for something that will make these weapons obsolete . . . . "

Reagan, hatless in a light rain, answered questions outside the White House as he departed by motorcade for the presidential retreat in Maryland.

Administration officials are concerned that Soviet Politburo member Mikhail Gorbachev had made progress during his European trip this week in convincing western leaders that the antimissile proposal, known as the Strategic Defense Initiative, stands in the way of a U.S.-Soviet arms control agreement. Gorbachev, widely believed to be the No. 2 man in the Soviet hierarchy, told the British Parliament that an arms race in space "would not only be dangerous in itself, it would give a boost to the arms race in other areas."

Asked about Gorbachev's criticisms of "Star Wars," Reagan said the Soviet leader "doesn't know what he's talking about."

"He doesn't understand exactly what it is that we're researching, but we're going to be pleased to let them know exactly what it is . . . ," Reagan said.

U.S. officials have made it known that the president's interest in a strategic missile defense system will be expressed when Secretary of State George P. Shultz meets with Soviet Foreign Minister Andrei A. Gromyko next month in Geneva. U.S. officials also have indicated that the system would be a subject of negotiation in the arms control talks expected to follow the Shultz-Gromyko session and have hinted that the United States might be willing to trade the proposal for Soviet reductions in offensive weapons.

A senior administration official who briefed reporters yesterday morning on the visit by Thatcher took issue with reports that she and other western leaders have reservations about the Strategic Defense Initiative.

"We and our allies are on the same wavelength," he said.

Reagan also was asked yesterday whether he would "get tough" with Japanese Prime Minister Yasuhiro Nakasone on trade issues when the two meet in Los Angeles Jan. 2.

"How can I get tough with a very good friend?" Reagan asked.

The president said Nakasone was "being most cooperative" and that Japan and the United States "have made great progress" in resolving their differences. "But there's a long way to go yet, and he knows that, too," Reagan added.

Reagan also was asked whether he would enjoy "a second honeymoon" with Congress when he returns from a week's stay in California. "If I've had a honeymoon with Congress, romance has been dead for four years," he replied.