"Did someone forget SIA whilst briefing the press?"
Shed, am I right in thinking it was only in the report of Cameron's comments that this mistake occurred? If so, it may well be that Cameron was wrongly briefed by a civil servant - would that surprise you - after all SIA did have rather a fight against bureaucracy getting a flight to Manchester in the first place all those years ago! Of course, it could be he was misquoted, but either way somebody didn't do their homework.
It remains to be seen what happens with the Singapore service but let's hope we retain what has been, as you rightly say, a very loyal customer after 28 years of operation.
Anyway, congrats to MAN for securing the HK route and for ensuring it received due publicity.