South African Bishop Desmond Tutu said yesterday he is "thrilled" with American demonstrations against his country's apartheid system, but suggested the best way to change the system might be for U.S. officials to practice their religion. Tutu, the 1984 Nobel Peace Prize winner, spoke at a UCLA rally in Los Angeles. He charged that Reagan administration policies are encouraging white leaders in his country to resist change.

He noted that many officials in Washington are professing Christians and said, "If they took seriously their faith, things would be different.

"Christians must be on the side of the poor and the oppressed, because that's how God is. The church has no choice. It has to be biased on the side of those whose noses are rubbed in the dust." 'Ronald Merrick' Returns? -----

Tim Pigott-Smith, the reviled Ronald Merrick in "The Jewel in the Crown," hopes to appear in a one-man play in New York or Los Angeles. The play, "Bengal Lancer," written by William Ayot, is based on the life of an Englishman who lived on India's northwest frontier at the turn of the century.

"I read this man's autobiography when I got back from making 'Jewel' in India," said Pigott-Smith, "and so many of his experiences and reactions to India tallied so exactly with my own that I thought I should do something about it."