I can well believe that it's for the tsunami. I can also well believe that it's done at that point because the aircraft is itself carrying the most accurate manifest.
What is a bit puzzling is why, in the 21st century, an electronic copy of the manifest can't be transferred to London and printed there for the police to check before the aircraft arrives. After all, a SIN-LHR flight will have been in the air for 12-14 hours by then.