Why the length of this road matters
An apostille issued in Brighton for use in Sri Lanka is not accepted, so this corridor ends with consular legalisation rather than with the British certificate. That extra stage is not merely another fee. It is another set of hands, another queue and another journey in each direction, and every one of those adds days during which the paper is somewhere other than with you.
Most guidance treats that as a scheduling problem. It is also a risk problem. The longer a sheet is in circulation, the more chances it has to be delayed, misfiled or lost, and what that costs you depends entirely on whether a second copy of it can be made. So the first sorting job is not by urgency or by importance. It is by replaceability.
The pile you can order again
Births, marriages and deaths registered in this district sit behind a permanent entry. Brighton & Hove Register Office at Town Hall, Bartholomew Square, Brighton BN1 1JA issues certified copies drawn from that entry, and each copy is a full official sheet rather than a duplicate of another sheet. If one goes astray, another can be made on the same footing.
This is why a certified copy is the sensible traveller. The sheet already in your possession stays at home and keeps its value, while a copy of equal standing goes on the long road. The price of that copy buys you a paper whose loss is an inconvenience rather than a setback, which on a route with a consular ending is a fair trade.
The pile that exists only once
Other papers have no open register behind them. A qualification from a body that has since changed hands, a court document, an award letter, or any record created outside the country can be very hard or impossible to reproduce. Nothing about the certifying chain distinguishes these sheets from any other, and no stage in it will treat them more carefully because they are irreplaceable.
So the care has to come from you, before anything is posted. Work out who would have to issue a replacement and whether that route still exists. If it does, the paper quietly rejoins the first pile. If it does not, you are making a deliberate decision to expose a unique sheet to a long journey, and it is better made knowingly than by habit.
What the sorting changes about your plan
It changes the order of work. Certified copies are ordered first, because they take their own time and because they define what the rest of the file looks like. Then the British stage runs on whatever sheets you have chosen to send. Knowing what you need for an apostille for Sri Lanka would not answer this, since the certificate that word names is not the one this road ends with.
It also changes what you keep. Note down what went into the envelope, in what quantity, and on what date, and keep that note with the sheets you held back. An apostille from United Kingdom to Sri Lanka is not the ending here, so the file will pass through more hands than usual, and your own list is the only version of events that stays in one place.