Alot of assumptions there. There will always be a contingency plan when it comes to live programming and a presenter becoming unavailable at late notice or during the bulletin. It's not so bad for the earlier bulletins as generally the later presenters are already around by the time the preceeding bulletin goes to air, and I guess in ITV's case they always have the London presenter around for News at Ten anyway.
It doesn't happen that often though - can't think of any examples other than the Ford/Raworth switch mentioned earlier, though it did happen on Breakfast earlier this year when one host fell sick, but not too much of a problem when you have a co-hosted format.
I preferred the internet when it had a sense of humour.