Why the run comes in separate parts
Turkmenistan has not joined the Hague Apostille Convention, so a British mark by itself does not answer for a paper used there. Documents going that way need embassy or consular legalisation instead. No apostille issued in Bristol for use in Turkmenistan finishes anything on its own, and each further stage is a fresh dealing with a fresh body that has never seen your papers.
Nobody is holding the whole thing
It is natural to picture an application making its way along a pipe with somebody watching over it. There is no pipe. An apostille from United Kingdom to Turkmenistan is one finished dealing, after which the paper is back on your table, and the next dealing starts only when you start it. Each body completes its piece and moves on.
That has a sharp practical edge. No office will tell you the run has stalled, because none of them knows a run exists. The only continuity here is the folder you keep and the attention you pay, and from the outside a week lost between stages looks exactly like a week of somebody else's work.
The three dealings, and where each ends
Bristol Register Office at The Old Council House, Corn Street, Bristol BS1 1JG prints certified copies of birth, marriage and death entries, and its part ends when the copy is handed over. The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX works by post, and its part ends when the sheet is returned. A consular desk is the third and last.
Sending a certified copy rather than your own original counts for more on a route with this many handovers, because each one is a chance for a paper to go astray. A civil record can be printed again by the register office. The original in your drawer cannot be conjured back out of a lost envelope.
What the separate parts cost
The British stage is £45 / document by standard post, or £40 / document next-day where the sender is a registered business, and an official charge arises again at the consular stage. Nothing is bundled and nothing carries over between stages, so the total is simply the parts added together, met one at a time as the paper moves along.