Which step comes first
There are two separate errands hiding inside this job. One is securing the piece of paper you are going to send. The other is having English written from it. They look interchangeable and they are not, because the second is written against the first. Reversing them is the most common way people end up paying for the same pages twice over.
The language United States requires
English is what documents used in the United States need to be in, so anything in your file that was issued in another language is the part requiring work. That is worth establishing early too, since it determines how many pages enter the second step and therefore what the second step is going to involve.
Certification the United States accepts
A certificate of accuracy is signed and dated by the translator and travels attached to the English. Because it refers to the document supplied, it is tied to the copy that was actually sent for work. That link is the practical reason the sequence matters: change the underlying paper afterwards and the statement no longer points at what you are posting.
Per-page cost and turnaround
The figures are £25 for each page and 2–3 business days from a clear image, with a 24-hour route when the date demands it. Both are easy to plan around once the paper question is closed. Notarisation can be added afterwards if the receiving body wants it, which is a further reason not to leave the underlying document unsettled.