There is no one to ask on the other end
Once the file reaches Afghanistan it is on its own. The person reading it has your paper, and nothing else. They cannot see the covering email you sent, they do not know which of two people with your name you are, and they will not call Plymouth to check. Everything the reader needs has to be visible in the Persian text itself, in plain form, without a hint of guesswork.
Persian, and every box on the page
Afghanistan works in Persian, so the whole record is rendered in Persian and not just the sentences. A record is mostly small fields: who issued it, under which entry, on what day, and who signed. Those are carried across in the same order they sit on the paper. An English page with a short Persian note stapled to the back is a different object, and it does not do this job.
Start from a copy you can afford to part with
The safest source is one you did not have to risk. Plymouth Register Office holds the civil records for the city and issues certified copies of births, marriages and deaths, so a second copy can be ordered and used as the working paper. The original then stays in your drawer. It is a small step that removes the one loss you cannot undo, because a lost certificate is replaced only by starting the whole errand again.
What it costs and how quickly it moves
The price is arithmetic rather than a quote: £25 for each page of the record, so a two-page file is two. Standard turnaround is 3–5 business days from a clear scan, and 48 hours is available when a date is close. Note that the clock starts at the scan, not at the phone call, so the fastest thing you can do today is get a legible image of every page in front of the translator.