Tony Johnson, the Yale University rowing coach, appeared more dazed than happy after his junior varsity eight had swept to victory in the Ladies' Challenge Plate championship match of the 129th Henley Royal Regatta today on the River Thames

Twenty-one years after he first visited Henley as an oarsman for Washington-Lee High School of Arlington, Va., Johnson owned his first title in his fifth appearance at this prestigious regatta. $"I guess in some ways I feel emotionally drained," said Johnson, making his coaching debut here after last competing as a rower in 1966. "We've had 10 races with Yale crews in the last three days. It's tiring. You have to keep getting prepared for another race."

Then, breaking into his first smile of a postrace interview, Johnson, who coached Georgetown University from 1967-69, said, "I'll be pleased all the way home."

Yale's varsity eight met its match today, losing to a combined eight from the London and Thames Tradesmen's boat clubs, which also serves as the British national eight. The British crew won by 2 1/3 lengths in 6:35 for the one-mile, 550-yard course.

The Yale junior varsity, which needed to win two races today, had four members from last year's Eli freshman eight which lost the 1978 Ladies' Plate final.

Stroked by David Potter of Vienna, Va., the Yale JV took a small lead from the University of London at the atart of today's semifinal and held off rpeated London attempts of the lead. Yale never relinquished its advantage, but never led by more than a third of a length until a sprint at the finish gave the Elis a half-length decision in 6:54.

The final was almost an anticlimax as Yale stroked 40 times in the first minute and then settled to a 36 rating to lead by one length after a quarter mile over Downing College of Cambridge, England.

Yale slowly increased its margin to 1 1/2 lengths at the midpoint of the course, before Downing caught a crab just short of the mile mark. The Cambridge crew stopped rowing momentarily, allowing Yale to easily triumph in 7:14.

"We knew if we settled it down it was our race. Settle it down to about 35 (strokes per minute), row our race and power it out on them," Potter said.

"It was a pretty uneventful race."