The University of London advanced to the Wyfold Cup semifinals for uncoxed-fours today, defeating the Potomac Boat Club of Washington, D.C., by 1 1/2 lengths.

Potomac took the early lead but London pulled level after a half-mile and then grabbed a lead it never relinquished.

Meanwhile, the chances of an all-American final in the Grand Challenge Cup to settle United States supremacy were dashed by the British national eight.

Rowing under the club colors of Leander and Thames Tradesmen; the British won by a quarter-length over Cornell, the IRA champion.

The University of Washington advanced to the championship by downing the Irish Garda Siochana by a half-length.

Sunday's final will indeed be a challenge for the Washington heavyweight varsity. No American crew has won the Grand Challenge Cup for 18 years, and Washington's recent form against Leander-Thames Tradesmen isn't encouraging. The British beat them twice at last weekend's Nottingham Regatta.

Overall, American hopes faded considerably today. Only five United States entries advanced to the last day's races.

Two American entries who reached the finals may be forgiven if they seem dazzled by silver. Jim Dietz of New York Athletic Club must scull against Tim Crooks, a member of Britian's silver medal eight in Montral.

An even more imposing task faced Harvard's Chris Wood and Gregg Stone, whose British opponents in the double sculls are Mike Hart and Chris Baiillieu. Hart and Baillieu won the silver in the event at Montreal, have won the title here twice before and hold the Henley course record.

The other American survivors on a disappointing Saturday were the University of Washington's second eight, which advanced to the final of the Visitors Cup, and a University of Valifornia crew, which is in the Wyfold Cup semifinals.

In its race against Leander/Thames Tradesmen, Cornell followed its usual approach of holing back early and then trying to wear down the other crew with a late spurt. It might have worked under heavier course conditions -- Cornell had been hoping for the strong headwind of the previous two days -- but it wasn't enough today.

After half a mile, Cornell was behind by 1 1/2 lengths, according to the race umpire, although Cornell Coxswain James Howe and neutral observers claimed the distance was never more than a length.

Cornell then spurted, cutting the British lead to half a length at the mile post and to quarter a length at 1 1/3 mile. Cornell got no closer than that.

Among the Cornell cre w is Chip Lubsen of Alexandria, Va., who rowed for T.C. Williams High School and was a member of the Potomac four that won the Stewards Cup at Henley in 1975.