Originally Posted by
oliver2002
The immigration system of the destination also checks the documents presented. For example to see if the passport has been reported as stolen. Just having the passport of the destination country doesn't really matter.
The UPT documentation I found is not entirely clear, but I'm leaning towards the interpretation that "ok to board" status does not need to be queried for UK nationals and is presumed.
Of course, immigration authorities will run a myriad of other checks based on PNR data, but that's not related to check-in.