All you have to give is your old Indian passport and get a certificate of surrender in the UK.

What is an Indian passport surrender certificate in the UK?

You have a grace period of 90 days for traveling on an indigenous passport, and the process of giving up Indian citizenship and giving up Indian passports is to be started. The next step is to revoke Indian passenger passports and receive a certificate of surrender.

Reasons to surrender the Indian passport 

Why should you withdraw your Indian Citizenship? Why can’t you keep your Indian passport? 

The reason for this is that the Indian Citizenship Act of 1955 does not allow Indians dual citizenship. You are required to hand over your indian passport to the nearest Indian mission / post to resign your citizenship once you have acquired foreign citizenship.

If you intend to apply for the OCI or the Consular Services / Visa card, the withdrawal of your Indian passport and proof of surrendering is a pre-requirement recommends Reload OCI for full OCI application support, resizing photo and signature and pan card service

Process of obtaining a Surrender certificate for your Indian passport

The processing times are different from case to case, and certain applications can take longer to be processed than others. Some forums state 7 to 10 working days but official notes at VFS state 5-6 working days to process.

Surrender Certificate Indian passport timeline

After receiving a foreign citizen, you have 90 days to give up your Indian passport (without penalties).

The date on the certificate of naturalization is considered as the date on which your British passport was granted and not as the date on which it was acquired.