Britain's Conservative government, still buoyed by last spring's Falklands triumph, is pledged to give the highest priority to the country's defenses, "both conventional and nuclear," Prime Minister Margaret Thatcher declared today.

Addressing the closing session of her party's annual conference, Thatcher stressed Britain's continuing nuclear role, its support for NATO and a "strong united Western alliance." In the aftermath of last week's overwhelming vote by the opposition Labor Party in favor of abolishing the country's nuclear arsenal, Thatcher asserted that "having nuclear weapons makes peace more secure."

She noted that Labor had voted to remain in NATO and "shelter behind American nuclear weapons," but oust U.S. nuclear bases from Britain. That, she said, was "like expecting an insurance policy while refusing to pay the premium."

Thatcher also asserted that Britain remains a "reliable ally" and "a friend to be trusted," a message presumably intended for the Reagan administration, which has sharply disagreed with Thatcher over the U.S. ban on equipment for the Siberian natural gas pipeline.

The speech, Thatcher's most important since the Falklands conflict, capped a week in which she was repeatedly lionized by party faithful meeting at the seaside resort of Brighton. The war had been won by courageous British men and one woman, said her defense secretary, John Nott. A resolution put forth by a Manchester party group called her "full of backbone and guts."

Using an American-style teleprompter, Thatcher spoke for 45 minutes and got a six-minute standing ovation from her party. Her main theme on domestic economic issues as well as foreign policy was firm adherence to present lines, no matter what the consequences.

On Britain's highest-ever unemployment figures and continuing recession, she observed, "The anxious say 'you cannot do everything at once . . . why not adapt your approach a little, give in for the time being till things are getting better; then you can start again.' "

"To do that," she said, "would be betrayal." Her government's main asset after three years, Thatcher declared, "is constancy and resolve."

Thatcher's term runs out in the spring of 1984 and an election may be held as early as next fall. Today's highly political speech plainly marks out the ground for the campaign, whenever it comes.