A stamp belongs to a route, not to a document
It is natural to think of a certified document as finished — done once, good everywhere. That is not how any of this works. Each mark on the page answers a question posed by one particular destination, under one particular arrangement. Change the destination and you change the question, so a sheet that satisfied one country tells the next one very little.
Where Cambodia stands
Cambodia has not joined the Hague Apostille Convention, so papers intended for Cambodia need embassy or consular legalisation instead. There is no apostille issued in Belfast for use in Cambodia that a desk there will act on, however recently it was added and whatever it cost. That is a position taken by the country, not a comment on your paper, and it applies to a spotless new certificate exactly as it applies to an old one.
What to do with a mark already on the sheet
Leave it. An earlier certificate is not a defect and nobody in the chain will treat it as one, so there is no cleaning-up stage to arrange and nothing to reverse. The only cost is cosmetic and practical: a page already carrying stamps, staples and a fastened certificate has less room on it, and a crowded sheet is slower for anyone to read.
Starting again without starting from nothing
Some of the earlier work does survive. The names you settled on, the list of what the far end wanted, and the knowledge of which body issued each paper all carry forward untouched. What does not carry forward is the certification itself. An apostille from United Kingdom to Cambodia is not on offer, so the route has to be rebuilt from the document upwards rather than resumed from where the last job stopped.
The office behind the stamp, and what it charges
The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX issues apostilles for the whole of the United Kingdom, working by post, with no counter to visit in Belfast or anywhere else. Anyone searching for what you need for an apostille for Cambodia lands on that office and then discovers it is the wrong turning here. Its published charge still gives you a sense of scale for the road that does apply.