Caster Sugar Substitute
Caster sugar is just ordinary table sugar that has been ground to a finer texture. It is still grainy like sand rather than completely powdered. 

Bura, on the other hand, is caramelized and dehydrated. It is typically made by dissolving table sugar in water, then boiling off the water. This results in a texture like that of caster sugar, but a deeper flavor. 

Generally sugar in a recipe provides more than sweetness. The texture of the end result is affected by the sugar. Since bura has less water than regular caster sugar, it will probably result in a drier scone. 

Also, part of the taste of any baked good comes from the caramelization of sugar that occurs during the baking. Since bura is already caramelized, it will be more prone to burn. 

Finally, bura is intended for desserts like laddus, where sugar makes up a much higher proportion of the ingredients and the overall flavor is supposed to be sweet. Why use it for something like a scone? Sweetness is an important contributor but not the main goal for scones, and there is no point using something as complex as bura when its flavor profile will be overwhelmed by the other ingredients. 

For these reasons, I think you are better off just grinding ordinary table sugar rather than using bura. If you don't want to go through the bother of grinding, I think you'll have better luck using the sugar as is instead of using bura; though that isn't a great choice either.