One word, two papers
Forms use the word certified for two quite different things. One is a copy: a fresh print of a record, issued and stamped by the office that keeps it. The other is a translation: new text in a second language, signed by the person who wrote it. They come from different places, cost different money, and prove different things.
The copy a register office issues
Southampton Register Office, 6a Bugle Street, Southampton SO14 2AJ holds births, marriages and deaths registered in the city. It will issue a fresh certified copy of one of those entries, which is why your own certificate never has to leave the drawer. What that office cannot do is put a record into a second language, or say anything at all about a paper it did not register.
The paper a translator signs
A certified translation in Southampton for India is the other artefact. A named certified translator produces the English and attaches a certificate of accuracy: a short dated statement saying the English carries the content of the sheet it was made from. The signature on it belongs to that translator alone, and to no register or public body.
Working out which one you were sent for
Look at what the wording actually names. A line about a copy of your certificate is answered by the register office and nothing more. A line that mentions language is not: it wants an official English translation in Southampton, made from whichever sheets are not in English. Plenty of people buy one, arrive with the wrong paper, and begin again.
What each one costs you
Fees at a register office are its own affair and sit outside this price. The language work is about £25 / page, with 2–3 business days as standard and 24 hours on the express route. Buying a certified translation into English in Southampton for sheets already written in English is the usual way to overspend on this errand.