I know you're not supposed to be able to get Rupees outside India, but I'm sure I heard someone say that you can get Rupees at Heathrow Airport - can anyone confirm this?
The best way to get Rupees in India is from an ATM. Changing money in banks can take hours as the bureaucracy is endless.
Ask for plenty of small denomination notes (10 and 20 rupees) so you are ready for tipping (which is constant in India).
Euros, US dollars and Sterling are accepted and easily changed at most hotels. Almost all types of credit cards are accepted by larger restaurants, hotels and shops.
Most banks charge you a fee for withdrawals from an ATM outside your home country - the Nationwide (UK) does not charge, but I pay £1.50 per withdrawal with HBOS/Halifax!