Chile reads the pair, not the page
Chilean offices work in Spanish, and they rarely look at a translated sheet on its own. The English record and the Spanish version are read as a pair, one against the other. That habit is why a certified translation into Spanish in Birmingham is built to match the source rather than simply carry its meaning.
Your English paper is never altered. No stamp goes on it, and it does not change hands. The Spanish is a second sheet, produced from a scan, and it exists so that a reader with no English can follow the original line for line without ever needing to guess.
How the mirror is made
The translator maps the source before writing a word: what sits in the header, where the body text runs, what is printed down the side, what is stamped across the bottom. Every one of those gets a home in the Spanish. Nothing is silently dropped because it looked like decoration.
Numbers, names and reference codes are carried across character by character. A registration number is not rounded, tidied or reformatted. Where a Chilean reader spots a digit on the English, the same digit is waiting in the Spanish, which is the fastest way for them to satisfy themselves and move on.
The signature, the price and the source sheet
Beneath the Spanish sits a certificate of accuracy, signed and dated by the named translator. That is the certified form a certified translation for Chile takes, and it is what turns a good rendering into a document an office will act on. An official Spanish translation in Birmingham without that signature is just a helpful text.
Start from the cleanest source you can get. Where the record is a birth, marriage or death entry, Birmingham Register Office on Holliday Street issues fresh certified copies for an official fee, so the sheet you keep at home never has to travel and the scan is sharp enough to mirror properly.