OCI application is a two step process
Step1- Application Submission
Step-2 OCI Collection
Both the steps can be done in post/personal. The suggestion is to do in person especially Step 1, as they are experiences where HCI lost some of the applications and you don't want yourself to get into that situation.
You can apply OCI to your nearest embassy, ex- Birmingham, London, Edingburgh.
To apply OCI first you have to Surrender Indian Passports and obtain Surrender Ceriticates without which you cannot get OCI
Once you have the Surrender certificates, please fill up the form online at the below link
https://passport.gov.in/oci/
Please fill the application as per the details you have mentioned in Indian Passports example, father name, mother name, spouse name which you never have in British Passport.
Once the application is filled in, make sure you answer all the questions and then submit, upon submission take the pdf print out in one set and attach the following
1. photographs 51*51- 1 per person
2. British Passport photo copies all details and observation pages-1 per person
3. Marraige certificate-1
4. Child full version Birth certificate-1
5. Surrender certificate photo copy Plus Original-1 per person
6.Indian cancelled passport-1 per person
7. fee-175£ per person
submit either in person or post and wait for 1-2 months if London else 3-6 months for other consulates