Harrison, who had already medalled at the competition and at the previous week’s British Summer Championships, took victory in the 17-year-olds’ 100m backstroke. He clocked 58.57 seconds in Saturday’s final, which also saw Leander’s Harry Middleton finish fourth in 59.92.
Harrison also reached three other finals on Saturday. He just missed out on another medal in the 17-year-olds’ 200m butterfly. He finished fourth in a race that saw the top four separated by just 0.7 of a second.
His Mount Kelly team-mate Tom Brew took second in 2:08.47, with Harrison touching in 2:08.96.
Harrison also came fifth in the 50m breaststroke in 30.52, with Leander’s Eanna O’Hara finishing seventh in 30.60.
Showing his versatility, Harrison also reached the 17-year-olds’ 400m freestyle final, where he came ninth at the end of a busy day in 4:19.25.
Cutler’s gold medal came in the women’s 17 and over 100m backstroke. She clocked 1:04.07 to collect her second title of the championships, having won the 50m backstroke crown the day before. Leander’s Imogen McCalman finished ninth in the 100m final in 1:06.81.
Plummer claimed gold for Leander in the 15-year-olds’ 50m breaststroke. He clocked a time of 31.50 seconds at Ponds Forge.
There was a bronze medal for Mount Kelly’s Molly Francis in the 16-year-olds’ 100m freestyle. She clocked 59.38, with team-mate Lauren Hession finishing 10th in 1:01.19.
Plymouth Leander’s Reid Jones again just missed out on a medal, finishing fourth in the 15-year-olds’ 200m butterfly in 2:17.00.
Also taking fourth was his team-mate Alex Maticiuc in the 12/13-year-old girls’ 50m breaststroke.
Maticiuc clocked 34.87 seconds, with Dinnaton’s Emily Morgan-Hughes, who had won bronze in the 100m, taking sixth in 35.06 and Mount Kelly’s Olivia Lee coming ninth in 35.71.
Fellow Mount Kelly swimmer Bella Hodges came sixth in the 12/13 100m backstroke, while team-mate Charles Hodges was sixth in the 16-year-old boys’ 100m backstroke.
Leander’s Oriana Noel also reached a final on Saturday, finishing sixth in the 12/13 200m butterfly.