The one thing the British desk checks
The FCDO Legalisation Office, PO Box 6255, Milton Keynes MK10 1XX, has one job. It matches the signature or seal on your sheet against a specimen it holds, then attaches a certificate of its own saying so. That is the entire British stage. It works by post and covers the whole country, so there is no counter in Coventry to attend.
That narrowness explains nearly everything else here. What you need for an apostille for Afghanistan starts with a paper carrying a mark the office can recognise: a certified copy from a register, a court seal, a signed company record. A plain printout carries nothing to match against, so it cannot enter the chain at all.
Four things no British desk will do
It will not read your document or judge what it says. It will not correct a name, a date or a place. It will not decide which route your paper takes abroad, and it will not add the consular mark that Afghanistan looks for. Every one of those sits with somebody else, and none of them comes with the fee.
The last of the four is the one that costs money. An apostille issued in Coventry for use in Afghanistan is not something the Legalisation Office can hand over as a finished article, because the destination does not work that way. Pay for the British stage alone and you hold a sheet that has been checked but still cannot be used.
Why Afghanistan brings in a second desk
Afghanistan has not joined the Hague Apostille Convention, so the shortcut that many countries share does not reach it. Papers heading there need document legalisation for Afghanistan instead: the British certificate first, then a consular stamp from the desk acting for the country. Both marks belong on the sheet before it leaves.
The order is fixed because each office builds on what the last one handed back. A consular desk works from a sheet the British side has already certified, so the two stages cannot run side by side or swap places. An apostille for Afghanistan, taken by itself, would leave the file one stamp short of usable.
The Coventry end of the work
Coventry Register Office, Cheylesmore Manor House, Manor House Drive, Coventry CV1 2ND, issues certified copies of birth, marriage and death entries registered in the city. Ordering one keeps the certificate you already own safely in your drawer, and it means a mishap in the post costs a fee and a wait rather than a long trail to replace something irreplaceable.
The British stage is priced per document: £45 by standard post, or £40 per document on the next-day service open to registered businesses. The count of sheets you settle in Coventry therefore sets the bill. An apostille from United Kingdom to Afghanistan is not sold as one finished product, and the consular desk charges separately for the stamp it adds.