Pope John Paul flew back to Rome tonight at the end of a tiring six-day, 1,000-mile tour of Britain after being greeted by the placards of teen-agers in Wales saying "Rock on, John Paul," "The pope is number one" and "John Paul, superstar."

The placards summed up the impression that the pope has created among Roman Catholics, other Christians and the large number of nonbelievers in Britain.

"He has come to Britain with the grace of a pilgrim and a prophet," said the archbishop of Canterbury, Robert Runcie, leader of the world's 65 million Anglicans.

"He has conquered hearts by his attentiveness and humility. He has spoken convincingly of the things of God, but has adapted himself to people and occasions with the sensitivity of a gifted pastor."

Looking tired but happy, the pope ended his tour in Cardiff, the biggest city in Wales, with an open-air mass attended by about 110,000 people--and a meeting with about 40,000 excited young people at a football stadium.

As on previous occasions, the pope mentioned the conflict between Britain and Argentina in the Falkland Islands, but today he added a reference to the fighting between Iran and Iraq.

"Let us . . . remember those who have died in conflict throughout the world--in the conflict in the South Atlantic, in the conflict between Iran and Iraq, in every place where human blood is shed," he declared.

One of the placards held high by the Welsh teen-agers asked the pope, who flies to Argentina in just over a week for a visit to balance his British tour: "John Paul II--take our love to the Argentine youth."

The pope did not have any official meeting with Prime Minister Margaret Thatcher or other government leaders, at Thatcher's suggestion, to avoid the impression that he supported Britain in the Falklands dispute, which would have angered Argentina's Catholics.

In his airport speech before leaving for Rome, the pope thanked Queen Elizabeth and the "civic authorities" as well as church leaders for his visit but made no mention of the British government.

At the open-air mass in Cardiff, the multilingual pontiff tried his hand at saying a few sentences in Welsh, a complex Celtic language. One expert described his accent as "interesting" and another said it was "a very good effort and better than many Welsh learners."

His homilies today were devoted to religious themes. "May it be recorded that I, Pope John Paul II, came to Britain to call you to Christ, to invite you to pray," he told the cheering teen-agers. At the open-air mass, where the pope celebrated the First Communion for young people, his theme was the value of the Eucharist.

His meeting with young people evoked an outburst of enthusiasm and affection matching the greeting he received from Scottish young people in an Edinburgh stadium on Monday night.

They sang "He's Got the Whole World in his Hands" and chanted "John Paul, John Paul" and at one point gave him a five-minute ovation.

In his sermons throughout the tour, John Paul has taken a positive attitude on the church's teaching--in contrast to the somewhat stern style he adopted on some other tours--but he has not softened Roman Catholic policy.

He made enthusiastic comments about the need for unity with other churches, but gave no indication that the Roman Catholic Church was about to change any of its positions to find common ground with the Anglicans.

While quietly reaffirming the indissolubility of marriage, he underlined his admiration for those who comforted and helped those whose marriages had broken down. While warning young people about the dangers of premarital sex and of drugs, he stressed that if they placed their lives in the hands of Jesus he would "make such use of your lives as will be beyond your greatest expectations."

As in Manchester and Liverpool on Monday, the crowds in Cardiff did not reach the size expected by church officials, but they nevertheless were enormous when compared with other meetings organized in the past by British churches.

Cardinal Basil Hume, leader of the Catholic bishops in England and Wales, probably spoke for many non-Catholics as well as the Catholic community in a telegram sent to the pope on his return to Rome: "You have won the admiration and love of millions of people in our country. . . May God uphold you in future pastoral journeys."