When we came to know about moving to London, the first thing I asked my mom to do is to prepare good molagapodi and sambhar podi. She is an expert in preparing these essential dry powders and even after marriage she gives me the podis regularly. Kindly check the recipe for authentic tambrahm idli podi recipe here. You can also try Podi Dosai using freshly ground gun powder.
I have used hot idlis however you can also prepare this from leftover idlis.