Combine butter, sugar, golden syrup, and cocoa powder in a saucepan over medium-low heat; cook and stir until smooth, 2 to 4 minutes. Stir in biscuits and raisins. Pour mixture onto a rimmed 8 x 12-inch jelly roll pan; press to flatten.
Melt milk chocolate and dark chocolate in top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching, 3 to 4 minutes. Pour melted chocolate over base in the jelly roll pan.
Refrigerate until set, about 1 hour. Cut into 20 pieces.