I believe this thread should be moved to the Other Countries forum as it is related to Indian exit formalities for foreign citizens and not to British citizenship itself.
This old thread describes the process vividly. Worth going through it.
Broadly you will need an exit permit from the FRRO. A quick Google search pulls up
this link to use to apply for the exit permit, but the thread above suggests that there are further hoops as well.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.