SR is Self Raising Flour, don't you make custard over there?
oh well it isn't necessary to use custard powder, you can use cornflour or what you call cornstarch.
Most custard powders have a yellow colour to the finished custard and this gives the biscuits a yellow look, if you use cornflour the biscuits will be paler again that is all. I think custard powder has a cornflour base to it. It is fine like cornflour. From memory again, I think the colour is annatto and there might be a little salt in it as well but am not sure.
Hope this helps, it is difficult at times converting a recipe between countries I have found. Oh I have also made these biscuits using plain flour using the following recipe -
250gm butter (room temp)
4 tbsp cornflour or (custard powder)
4 tbsp icing sugar, sifted
1 ½ cups plain flour, sifted
Icing
1 ½ cups icing sugar
1 tbsp pure vanilla essence
2 tbsp butter (unsalted) room temperature
Cheers
Maybe1day