Idli rice is an Indian rice which is parboiled. Parboiled rice is pre-processed in order to reduce soaking time before grinding and it also has gelatinised starch which gives added texture. Short or medium grained rice is used because you need over 80% amylopectin (s starch) to make soft and plump idlis. Long grain rice contains only 78% amylopectin and therefore is not suitable. If you are unable to get idli rice, you can use a risotto rice such as arborio, carnaroli or vialone nano, but as this is not parboiled it will need to soak for about 4 hours longer.
Idli rice can be obtained in the UK from Spices of India and some Asian grocers. However, the minimum quantity currently available (2015) appears to be 5 kg, so you might find it more practical to use a risotto rice.