A goof involving the London Underground

Why does Barry walk to Victoria Station and take the District Line to South Kensington, then change to a Piccadilly Line train going toward Cockfosters? (It says Cockfosters on the train.) If he goes more that two stops on the Piccadilly Line, he's going around two sides of a triangle instead of taking a Victoria Line to Green Park for the Piccadilly. (Unless maybe the Victoria Line didn't exist in 1946?)