Preheat the oven to 350 degrees F (175 degrees C).

Combine butter, brown sugar, and golden syrup in a medium saucepan over low heat. Cook, stirring occasionally, until butter and sugar have melted. Stir in oats and raisins until coated. Pour into a 7- or 8-inch square baking pan; the mixture should be about 1 inch thick.

Bake in the preheated oven until the top is golden, about 30 minutes. Cut into squares, then leave to cool completely before removing from the pan.