The 18-year-old Paralympic hopeful clocked 24.00 seconds to seal his win in Bedford.
The City of Plymouth athlete had won a bronze in the 100m the day before.
His success in Bedford followed on from claiming the world under-23 title recently and a silver medal and a personal best at the Anniversary Games last weekend.
Meanwhile, Arnott’s City of Plymouth team-mate Jonathan Edwards reached the discus and hammer finals in Bedford.
The Commonwealth Games athlete finished sixth in the discus with a throw of 47.01 metres and came 11th in the hammer with 58.44m.