Why the run has two stages
Laos has not joined the Hague Apostille Convention, so a British mark alone 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 Laos closes the file by itself, which is exactly why a calendar has to hold room for the stage that comes after it.
The stage with a published shape
The Legalisation Office is a single central body issuing British marks, and it works by post rather than across a desk. So the block on your calendar is an envelope out, a wait, and an envelope back. Its charge is published as well. One route, one method, one figure — that is what makes this the part of the run you can plan with real confidence.
An apostille from United Kingdom to Laos does not move faster because you need it sooner, and the quicker of the two published rates is defined by who is sending rather than by how pressing the job is. Treat this block as a fixed width. It is the one measurement on the page that stays put when your circumstances change.
What sits on either side of it
Ahead of the fixed block is the errand you own. Where the paper is a civil record, Bristol Register Office at The Old Council House, Corn Street, Bristol BS1 1JG can print a fresh certified copy, so the original stays with you and the sheet that travels is one you could replace. That errand takes as long as you allow it to take.
Behind the fixed block is the consular stage, the widest unknown on the page. It carries an official charge of its own and a pace set well away from Bristol. Sizing it hopefully is the commonest way a plan comes apart, because the mistake becomes visible only when there is no room left to absorb it.
Building the calendar backwards
Start at the date the paper is wanted. Put the consular stage beside it with the largest allowance you can bear, place the British postal block ahead of that, and the certified copy ahead of that again. What is left at the front is your real deadline for deciding things. If nothing is left, the plain answer is that the deciding had to happen sooner.