Preheat your oven to 350°F (175°C). Line a baking dish or tray with parchment paper for easy cleanup later.
In a large bowl, melt the butter and golden syrup together. You can do this by gently heating the butter and syrup in a saucepan over medium heat, or in the microwave for about 30-40 seconds.
Stir in the demerara sugar, mixing it until the sugar dissolves.
Add the rolled oats to the butter-sugar mixture. Stir everything well so the oats are coated evenly with the syrup mixture.
Press the mixture into the prepared baking dish. Use the back of a spoon to flatten it into a smooth layer.
Bake the mixture in your preheated oven for 15-20 minutes, or until the top is golden and slightly firm.
Remove from the oven and let it cool for a few minutes. Once cooled enough to handle, cut the mixture into square biscuits. Allow them to cool completely before serving.