On the eve of his 10-day trip to Europe, President Reagan yesterday expressed new support for Britain in its struggle to regain the Falkland Islands, saying that it would be "presumptuous" of him to insist that the British seek a negotiated settlement.

"I know that both sides have lost men," the president said in an interview with foreign journalists. "But England responded to this, a threat that all of us must oppose -- and that is the idea that armed aggression can succeed in the world today."

Reagan's comments, and those of other government officials, left no doubt that the administration is rooting for British forces to succeed in their drive to capture the main Argentine garrison at Stanley before the president arrives in London Monday.

At a news conference earlier in the day, Secretary of State Alexander M. Haig Jr. was noncommittal when asked specifically whether the United States would like to see a British victory--like to have the fighting done with--before Reagan arrives in London for a meeting with Prime Minister Margaret Thatcher and an address to Parliament that will symbolize the U.S. commitment to the Anglo-American alliance.

But Haig said that the "ebb and flow" of the fighting could well change the Falklands situation before Reagan arrives. It was an unmistakable reference to what another administration official, speaking without attribution, termed "the growing conviction" that the British forces will provide a military solution to the Falklands dispute.

Reagan's comment about the Falklands conflict came during an interview by television reporters from British, French, German and Italian stations. He was answering a question from a British reporter when he made the remark about it being "presumptuous" of him to insist that the British stop fighting and negotiate. Haig earlier in the conflict urged the British to seek less than a total victory.

Reagan did say yesterday he hoped that peace could come before any more lives were lost.

"I think all of us hope and pray that no more blood needs to be shed or should be shed in arriving at a proper settlement," Reagan said.

In the interview, Reagan also predicted that U.S. interest rates -- a principal concern of the European, Japanese and Canadian leaders he will meet at an economic summit in Versailles this weekend -- will continue to drop.

But he built a domestic political message into this prediction, saying that "when we get another budget of the kind we had last year that shows continued reductions in the rate of increase of government spending here, we will see another drop in the interest rates before the end of the year."

Last Friday in Santa Barbara, the president was asked whether he would be able to tell the European allies that the right signals were now being sent to lower interest rates.

"Yes, I'll hope to change the subject," Reagan quipped in reply, "because I wouldn't want to tell them that the Congress acted irresponsibly."

Carrying out this theme yesterday, Treasury Secretary Donald T. Regan responded to a question about U.S. interest rates by referring to problems in other countries -- inflation in France and Britain, unemployment in West Germany, and a "weak domestic economy" in Japan.

Regan, speaking bluntly, ticked off a series of touchy monetary, trade and investment issues that will be confronted at the economic summit and said: "Trying to get all these things together in one package is going to be a neat trick."

One of the issues on which obtaining cooperation will be most difficult is the pending $25 billion Soviet natural gas pipeline intended to supply Western Europe. Reagan opposes the pipeline and extension of credits to the Soviets to enable them to build it. He has so far refused to lift export controls that would permit General Electric to sell powerful turbine engines to the Russians for use on the pipeline.

Haig, who is considered less concerned about the impact of the pipeline on European independence than is Reagan, said yesterday that the United States, while not changing its view, would not "fracture the back" of its allies over this single issue.

But Reagan, in his interview with the European journalists, returned to his familiar view that the Soviets should be punished for their interference in Afghanistan and Poland and for conducting "the greatest military buildup in the history of man" during a period of supposed detente.

"And maybe we need to get their attention," Reagan added.