Spanish is the language of the file
Mexico reads and files in Spanish, so an English record from Reading needs a Spanish text with it before anything can move. The English page stays in the bundle as the source. The Spanish follows its layout closely, which lets a reader put the two side by side and confirm that no line has been dropped.
The certificate and the credential
The accepted form is a certified translation carrying the translator's certificate of accuracy. Behind it stands a certified translator with ATA or equivalent standing. The certificate itself is short: who translated the file, from which language into which, and on what date. It is fixed to the set, never sent as a separate email.
Price, speed and what arrives
You pay about £25 per page of the source, with standard delivery in 2-3 business days and express delivery inside 24 hours. Three things arrive: the Spanish text, your scan and the signed certificate. A certified translation for Mexico is handed over as that set, in the order the pages were issued.