Check the shape, not the sentences
Everything that makes a document official survives as a shape: a header, a table, a signature block, a row of stamps along the foot of the page. Those shapes are reproduced in order, so you can hold the Khmer beside the English and see whether anything has gone missing. You need no word of the language to notice that a whole block is absent.
Counts and digits do the work
Numbers survive intact across writing systems, which makes them the most useful thing on the page for a reader who cannot read the rest. Compare the reference numbers character by character. Count the sheets. Count the stamps described. Where those three agree with your source, the set is almost certainly complete, and completeness is the part that tends to fail.
The side that looked blank
A sheet printed on both sides gets scanned on one side more often than anything else goes wrong here. The back of a certificate often carries a note, a stamp or the rest of an entry, and none of it can be written if nobody saw it. Scanning every reverse, even the empty-looking ones, closes the single most common gap in a bundle.
Cost and clock
The rate is £25 for each side as issued, so budgeting means counting sheets rather than guessing. Standard work takes 2–3 business days from a clean image, with 24 hours available on the express route. Where the source is a birth, marriage or death entry, Derby Register Office at Riverside Chambers, Full Street, Derby DE1 3AF issues fresh certified copies to work from.