The order of operations
Almost nobody wants a translation on its own. It sits inside a longer errand with a form, a fee, a deadline and often another stamp. The rule that saves the most money is plain: everything that marks the source page happens first, and the Spanish happens last. Reverse those two and the second job is simply a repeat of the first.
A snapshot, not a standing licence
The set describes one specific sheet of paper on the day it was scanned. It is not a permanent version of your document that can absorb later changes. Where a correction, an endorsement or a further signature is added afterwards, the Spanish no longer matches, and a reader comparing the two sees it at once. So settle the source, then render it.
Where to keep your slack
Build the spare time at the end rather than the beginning. The writing window is predictable; what is not predictable is the ten minutes you spend finding a discrepancy in your own name across two documents. A day of margin absorbs that easily. A file that lands on the deadline itself has no room for even a small fix.
The rate, and the window
The price is £25 for each side of paper as issued, so the estimate is a sheet count multiplied out. Standard work takes 2–3 business days from a clean image and express closes in 24 hours. Where the source is a civil record, Derby Register Office at Riverside Chambers, Full Street, Derby DE1 3AF issues fresh certified copies, which belongs at the front of the queue rather than the back.