If you are in in Chennai then you can speak to the staff at the British Deputy High Commission for assistance as a Brit. You can also contact the British High Commission in New Delhi for assistance as a British citizen. If you are wanting to apply for a visa such as a or a visitors visa for Britain then you can also call the Deputy High Commissioners office in Chennai.
You can also apply for a new British passport. Note the telephone number and a map to the offices in Chennai. Also note that there are a number of other Deputy high commissioners offices in India such as the British High Commission in Mumbai as an example.
British Deputy High Commission Chennai
20 Anderson Road