Please refer to the guidelines of the Reserve Bank of India:
https://www.rbi.org.in/scripts/FAQView.aspx?Id=11
The FAQs are fairly clear, and unless there has been some more recent update, the amount of Indian currency you can bring into India is restricted to INR 5000 or less. (point # 18 in the link).
That said, I don't think this is strictly enforced. It is unlikely that you will be searched for cash at the airport as you come in, unless there is something else about your luggage that interests customs officials and in the course of looking through your baggage they discover cash.
What rates are good? It is not easy to make a great deal on buying exchange if you are not in the business, but a simple way around is to check for the GBP to INR rate online and not fall for something ridiculously off the online rate. Most banks will exchange foreign currency, and there are many money exchange 'shops' in all cities. The latter usually offer a better rate. Among banks, non-Indian banks will probably offer a better rate.
Best to get about 75 GBP worth of INR before you get into India, and then exchange more cash in Cochin. It is a tourist friendly place, and there are plenty of places where you can legally exchange your money for INR at reasonable rates - but do compare rates before you exchange large amounts.