The route Sri Lanka actually uses
Take the route question off the table first. Sri Lanka has not joined the Hague Apostille Convention, so document legalisation for Sri Lanka runs through the consular chain: the UK stage, then a stamp from that country's mission in the United Kingdom. An apostille for Sri Lanka on its own leaves the paper half done. It is a common mix-up, because the same certificate settles matters for many other countries and the word travels further than the rule does.
Settle the number before you order
Every finished sheet ends its life in someone else's file. A university keeps what it is given, so does a bank opening an account, and so does an employer building a personnel record. None of them will lend it back to the next one. Work out how many separate readers you are dealing with, add one for your own drawer, and treat that total as the size of the order rather than something to revisit later.
The number is easiest to change at the very beginning and hardest to change at the end. A second run means a fresh certified copy, a fresh trip through the UK stage and a fresh consular stamp, all in sequence. So a request for an apostille from United Kingdom to Sri Lanka that arrives one sheet short is not a small setback. It restarts the clock on the whole chain for that extra sheet.
Running a set through together
A set moves as easily as a single sheet. Sunderland Registration Service issues certified copies of civil records, so you can order several at once and leave the certificate you already own at home, untouched. The FCDO Legalisation Office then handles the batch by post; it holds no public counter, and it treats each sheet on its own terms. What arrives back is a stack of separate finished papers, not one paper with several stamps.
What readers in Sri Lanka tend to ask for
Readers there differ in what they want to hold. Some accept a sight of the paper and hand it straight back, while others take it into a file and keep it for good. Where a family matter and a work matter run at the same time, assume the two are handled by different people who never speak. An apostille issued in Sunderland for use in Sri Lanka is not a transferable licence; it is a mark on one sheet, and the sheet goes where you send it.
Cost and timing for a set
Because the charge is per document, the cost of a set rises in a straight line while the waiting time does not. Three sheets sent together wait roughly as long as one sheet sent alone, which is the whole argument for deciding the number early. Knowing what you need for an apostille for Sri Lanka is really knowing your own list of readers. Build that list, order to it, and the route runs once instead of twice.