Tested, not merely claimed
Anyone can say they know Spanish. A certified translator has been examined on it, and that is the difference an official reader is looking for. ATA certification, or an equal qualification, is the mark this route relies on. It gives your pages a name, a body that tested that name, and a signature that can be checked. None of that depends on your own say-so, which is exactly the point.
Spanish is what Argentina reads
A certified translation into Spanish in London is required for papers that will be used in Argentina, whatever language they were issued in. An English sheet on its own leaves a clerk with nothing to act on. The Spanish becomes the version that is read and acted upon, and your English page travels beside it so a name, a date or a registry number can be matched straight back to the source.
What the signed statement actually claims
The certificate of accuracy is short, and every line of it does work. It names the paper translated, states that the Spanish is true to it, gives the translator's name and credential, and carries a date. From the moment it is signed, the accuracy of the text is that person's claim rather than yours. That shift is the whole reason the sheet exists and why it must stay with the set.
Rate, days, and the paper you keep
Cost follows paper: £25 a page, counted by printed sides, with standard work at 2–3 business days and express at 24 hours. A certified translation in London for Argentina never asks for the sheet in your drawer to travel, because the job runs from scans. Where the record is a civil one, a fresh certified copy from Westminster Register Office can stand in for the one you hold.