Do you have OCI or the erstwhile PIO card? Why do you need to reacquire Indian citizenship?
Renouncing British citizenship will not revive your Indian citizenship. It would make you stateless.
But, because the UK is a signatory to the Convention of the Reduction of Statelessness, it will not allow you to renounce your British citizenship unless you have got another citizenship.
So, if you are serious about the matter, start off the process for reacquire Indian citizenship. One way may be under Section 5(b) of
the (Indian) Citizenship Act 1955.
Also, can the mods move this thread to the Other Countries forum, as it is more relevant in that area?
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.