Yes. It's zero! The import of Indian currency into India is prohibited. There's only one exception - tourists returning from Nepal into India who took Rs with them, and who have underspent while in Nepal. Even then, any such currency has to be declared on re-entry. Sorry!
You can use their ATM machines or take currency, but some places i.e. Thomas Cook Exchange will only change bank notes that are "clean" i.e. if some previous owner in the UK has scribbled on the note they wont take it so check your notes before u go.
They will exchange small amounts at Heathrow & Gatwick but they do warn you its illegal [ thats what I do for taxis & tips on arrival.]
Some India aiports [MUMBAI] have a bank in the airport but be prepared to queue.