Double Olympic champion Rebecca Adlington added the 400m title to her 800m crown on the final night of the National Championships in Sheffield.
"It was a tough race," said Adlington. "Last night felt really strong but tonight I felt tired."
Sixteen more swimmers have now been added to the British squad for July's world championships in addition to the 21 who had already qualified.
The names of about 40 more swimmers for Shanghai will be unveiled on Monday.
The pattern of the 400m race was similar to the 800m final, with Eleanor Faulkner staying with Adlington until the champion pulled away with 150m to go, finishing in four minutes 08.32 seconds.
Adlington had already qualified for Shanghai in the 200m, 400m and 800m freestyle, topping the world rankings in the two longer distances.
Meanwhile, Jemma Lowe moved up to third in the world rankings with an impressive 200m butterfly, winning the race by 5.71secs in 2.05.36. It lifted her above fellow Briton Ellen Gandy, who had already qualified, in the rankings.
British record holder Simon Burnett booked his place in the 100m freestyle to add to his 50m berth. He beat Grant Turner by 0.23 seconds after touching home in 49.09.
Burnett, an Olympic finalist, said: "I am very pleased with that. It's been a tough few days, it may not seem that way to anyone looking but at 28 it adds up.
"I'm very happy with that, very happy with another win, and to swim well in my home country. Next up Shanghai."