Stir together 2 teaspoons soy sauce, sherry and sesame oil in a bowl. Add chicken and toss to coat. Cover and let marinate at room temperature for 20 minutes. Meanwhile, stir together water, rice vinegar, sugar, cornflour and remaining 3 teaspoons soy sauce; set aside.
Heat 2 teaspoons rapeseed oil in a large pan over medium-high heat. Stir fry chicken until nearly cooked through, about 5 minutes. Transfer chicken to a plate. Add chilli peppers, spring onions and remaining 2 teaspoons rapeseed oil to pan; stir fry for 1 minute. Add bok choi and ginger; stir fry 1 minute more. Add chicken and soy sauce mixture; cook until simmering. Sprinkle with peanuts and serve with rice.